Testing Hardness , Toughness and Stability of Gemstones

How Gemstones durability is analysed? 

Durability of gemstones helps in gems identification of particular gemstones through its particular characteristics or resistance power in particular environment in its internal structure or externally.  There are many experiments done on mineral stones and their results tell us about how much stone is hard or tough. There are normally following methods or criteria to check stone durability.

  1. Hardness
  2. Toughness
  3. Stability

Gemstones found in sand grains are more durable because they faced lot of resistance during transportation like weather, abrasion etc.

Examples:

Quartz, Topaz, Corundum, Spinel, Chrysoberyl, Diamond, Garnet

1- HARDNESS

The meaning of hardness is ” resistance to scratch”. When any gem is scratched with force by anything which is more harder than it, the appearance of no scratch indicates that the gem is harder. This is however destructive test and is not allowed by labs normally. In cabochon case, this test can be done at the back of cabochons. It should be done on rough crystals only or on carved pieces.

There are some standard sets in Moh’s scale of hardness which has distributed some gemstones in particular hardness value. This is not absolute but a relative test.

 

Quartz (7) ect but Quartz cannot scratch Corundum. Topaz is harder than both Quartz and Feldspar. Apart from there are some other hard material also as mentioned below:-

  • Steel—————–6.5
  • Kind Blade——— 5.1
  • Wire Nail———– 4.5
  • Penney ————– 3.5
  • Fingernail ———- 2.5

Some gemstones have two type hardness in one piece. For example Kynite has 5 hardness in parallel lines to the prism face and 7 hardness in perpendicular to this.

Kynite Crystal 

Jadite

2- TOUGHNESS

The meaning of toughness “ resistance to breakage“.  In practice this is considered to include resistance to chipping and cracking as well as actual breakage. Poly-crystalline gemstones are normally rough. Examples are Emerald. A stone may be tough or not tough even. Nephrite is not hard but extremely tough.  It should be kept in notice about following:-

Cohesion

All substances are considered to be composed of small particles called ATOMS. These minute particles are held together by a force of attraction called COHESION, which tends to resist any separation between the atoms. When a substance is broken it is done by a force that over comes this attraction. Thus the stronger the cohesion, the greater will be the effort necessary to break the substance.

Poly crystalline are tough because cracks affects individual gains or particles in a gem but finds difficult to cross from grain to grain or particle to particle.

Note: Mostly gems are crystalline. The few are non crystalline or poly crystalline. 

Elasticity

This is an ability of mineral to undergo a certain amount of distortion or pressure and finally it comes back to original form. Some minerals (metals) may be cut (SECTILE), hammered flat (MALLEABLE or drawn into wire (DUCTILE).

Sectile:

This type can face force e.g plastic

Malleable:

If material is stretchable, it can be drawn into wires and sheets e.g. copper, gold, silver.

Brittle

It refers to absence of ability to withstand distortion without separation. For example, the stone or minerals which are broken down or becomes power instead bending or flatting while struck with sufficient force. Such stones or minerals are called ” Brittle”.

Examples: Emerald, Opal, Fluorite, Amber, Some Zircon, Tanzanite and Moonstones. Brittle does not mean to be broken down easily but finally turns into powder like piece instead being flattened as copper or silver does. All gemstones or minerals are specified as brittle which are further divided into ” FRIABLE (turns into grains) ” SOFT” (easily to be powdered) and ” TOUGH” (offers a bit resistance to be separated).

Precaution:

  • Stones should not be kept in one bag because rubbing can damage to them.
  • Keep them oiled.
  • Dont put hard wire with stones in bag.
  • Dont put diamonds with emerald in 1 bag.

Cleavage

Cleavage is called straight crack which occurs due to weaker atomic zone of stone or part of stone. Weaker zones means that there is poor atomic arrangement having space between eachothers.

There are some very clear features of crystalline materials which are in the regularity of atomic arrangements. In some gemstones, spacing of atoms in different directions is the same. When such gem is broken, the breakage will be on the path which will relieve stress causing the breakage. In some gems, the atomic arrangements on some planes with wider spaces between these planes. In such stones, the breakage occurs parallel.  In some crystal direction on single crystal, atoms are closed togather. In some other directions, bonds are weak.

Cleavage can be of following types:-

  • Straight cracks / fractures
  • Irregular Cracks / fractures

Some points of Cleavage

  1. Occurs in crystalline material
  2. Can be internal / external
  3. Direction of splitting is dependent on internal crystal structure
  4. Cleavage can not occur in amorphous material as glass
  5. In Amorphous material, there are no structural planes where well defined splitting can occur.
  6. Breakage of glass is possible by fracture
  7. A single cleavage can not occur alone in some crystalline objects e.g. Jades
  8. In Diamond, all the atomic bonds are equal in straight but there are relative weaker zones parallel to certain well defined structural plan. It gives  OCTAHEDRAL cleavages with 8 facets.
  9. Cleavage can occur on flat surface.
  10. Cleavage may reflect or block the light due to internal reflection and you may see crack or minor flat like inclusions.
  11. Cleave which is not extended through out the crystal called incipient cleavage. it may show pearly luster e.g. topaz.

Cleavage can be defined in terms of quality and directions.

Quality

Perfect: Broad, flat, shiny surface

Good: less flat surface

Fair to Poor: Seldom visible in gem quality stones

Advantages / Disadvantages or Cleavage

  • Cleavage is quicker than sawing
  • Hardness direction can be found in diamond due to cleavage.
  • Facets can be polished to cleavage
  • Its necessary to cut facets at right angle to the cleavage plan.
  • In some cases, incipent cleavage makes the stone weak.
  • It can destroy stone during thermal shock or blow (pressure) heat, sudden stress while cutting and polishing or when repairing.
  • Cleavage can affect the durability of stones.

FRACTURE

The term FRACTURE is used to describe the chipping or breaking of a substance in any direction other than that of a cleavage plane. It may present in both crystalline / non crystalline materials.

Type of Fractures

Shell or CONCHOIDAL Fracture

Conchoidal is pronounced as kon-KOY-dal. If gemstone is broken in 2 parts, there leaves certain surface marking like sea shells. Which is called conchoidal fracture. It resembles on the ridges of sea shell. It occurs in glass or mostly transparent coloured stones.

Hackley Fracture / Fibrous Frame

Fiber material produce Hackley fractures i.e. jade. Fracture appears in fiber like patterns.

GRANULAR, SPLINTERY, UNEVEN AND EVEN

Some time when gems are broken, the surface of broken area appears like grains of sugar.

3- STABILITY

The meaning of stability in stones means “ability to resist reaction or change caused by heat, light, chemical attacks” .

There some kind of Causes which many damage the stone.

  • Heat —————————————– Pearls, Coral, Opal, Emerald
  • Thermal Shock —————————–
  • Light + UV Light / Irradiation———— Zircon, Kunzite
  • Chemical and Ultrasonic —————— Coral, Pearl, shell, Turquoise, opal,
  • Burning ————————————– Diamond

Some Notable points:-

  • Surface of diamond may burn if it is caused by heat in air. So it must be coated to protect surface during repairing.
  • Burning may reduce surface polishing
  • Glass filling in diamond or corundum may be melted during heat
  • Opal and amber may crack
  • Oil in emeralds may be dried out or removed.
  • Stones may fracture if they are suddenly heated or cooled too quickly during jewelry repairing.
  • Stone of poor toughness may be placed in ultrasonic cleavages

 

 

 

 

 

 

 

 

 

 

 

 

Some Videos:-

 

 

 

Testing of Gemstones for Identification through Polariscope

polariscope is an instrument in gemology for testing of gems with single refraction, double refraction, Poly-crystalline material or ADR.

Let us first tell you what is Polariscope? and how test is done?. The name of Polar – Scope itself shows or describes that a method to see the stones from poles or axis through the Scope. It helps in understanding different colour of light being transmitted by a gemstone. Polariscope basically is an optical device. A polariscope is composed chiefly of a light source and two crossed polarized lenses such as Polaroid {{Registered Trademark}}. One can determine both optic character and the optic sign of a gemstone. It helps in understanding synthetic, glass or natural gemstones.

In Polariscope, there are two lenses. One is on the top side called ” Analyser”. The other part is at the bottom which is called ” Polariser”. Above Polariser, there is ” Tint  Plate “.  Stone is put on the polariser and analyse through Analyser. Tint Plate plays role in rotation so gems view can be viewed by 360 degree.

How to Analyse through Polariscope:

There are brief description of two phase testing methods by polariscope that how gems are tested.

1st Phase of Test:-

Step:1 Rotate the analyser and check if there is complete darkness

Step: 2 Put gem on polariser

Step: 3 move the gem or rotate the Tint Plate at 360 degree and note down the light refraction spectrum on gems.

2nd Phase of Test for Further Confirmation:-

Step:1 Rotate the analyse and check if there is complete darkness

Step: 2 Put gem on polariser

Step: 3 Leave the gem in full light condition

Step: 4 now quickly rotate the analyser only one time and see final condition of light on gem.

Type of Identifications of Gems through Polariscope

By the above test, following results can be found for Gemstones that in what following categories they fall in:-

  • SR ( Single Refraction) / isotropic / Cubic crystalline
  • DR (Double Refraction) Anisotropic
  • ADR (Anomalously double refractive or an
  • Anisotropic Aggregate (Polycrystalline)

1- SR ( Single Refraction) / isotropic / Cubic crystalline

Observations:

Stone remains dark through 360 Degree rotation of Tilt Plate. Single Refractive of Isotropic gemstones gives constant dark light. In 2nd phase test, if gemstone appears bright light, it is SR. It occurs in mostly non crystalline gemstones.

Conclusions

Isotropic – amorphous or cubic

Examples

garnet, spinels, paste, natural glass, flourite, diamond, opal, plastics

2- DR (Double Refraction) Anisotropic

Observations:

Stone becomes light and dark four times through each 360 degree rotation in most orientation. In double refractive, gemstones gives changing light pattern. Sometime it appears as dark and some time it appears as light. This happens with mostly in crystalline gemstones.

Conclusions

Anisotropic – uniaxial  or biaxial

Examples

Beryl, Chrysoberyl, Corundum, feldspar, peridot, quartz, topaz,, tourmaline, zircon, tanzanite

  

3-ADR (anomalously double Refractive)

Observations:

Stone shows anomalous extinction effect. In this case, the gemstone gives anomalously double refractive. The pattern of the gems gives snake like dark and light both refraction. Smooth snake like refraction occurs mostly in Synthetic or glass stones.

Conclusions

Strain Anisotropy, often in otherwise isotropic material.

Examples

Almandine garnet, diamond, paste, natural glass, verneuil, synthetic spinel, some plastic, amber and some fire opals,

4-Anisotropic Aggregate (Polycrystalline)

Observations:

In this case, gemstone gives only bright light. Stone stays light through 360 degree rotation in all orientations.

Conclusions

Polycrytalline. Some twinned crystals, some doublets and triplets. Anomalous internal reflection effects.

Examples

Jadite, Nephrite, Chalcedony / Agate, Twinned sapphire, sapphire / synthetic ruby doublet, Reflection from cleavage in flourite.

Polariscope Tutorial by Nick Hodgson of Astrogems

 

Identification of Gems – Calcite dichroscope

Observing Gemstone colours through Calcite Dichroscope.

A dichroscope is a handheld tool for identifying or looking into the different colors or shades of a pleochroic piece of gemstones. It helps any gemologist or gem cutter in judging gemstone before cutting or identifying any gem.  

Calcite Dichroscope

Calcite Dichroscope is used to observe dichroic gems and trichroic gems regarding observing two different Colours in gem or three different Colours in gemstones. The gems with having strong difference of colors separately will be called Pleochroic.  It works because the calcite rhomb (Iceland spar) separates the polarized slow and fast ray emerging from the gemstone. If you look at the viewing end of the dichroscope, two small windows are seen. A gemstone is placed infront of Iceland spar with having white light in back side. Light enters through Aperture. Calcite rhomb separates the pleochronism clours in the gemstone. 
 
There has been taken in this picture, a close look of emerald through Samsung mobile camera by zooming it. If you see, this emerald is showing two different Colours. One is green and one is yellow green.
 
I will upload another video showing garnet plane colour.

HOW IT HELPS

_________________________
1—This helps in identification of gemstones.
2—It shows the true Colours of Pleochroic gem along each axis. It helps in cutting.
3—A gem cutter can judge the strongest colour and then he cuts accordingly for the best results.
 

HOW CAN YOU IDENTIFY GEM:

_____________________________________________
…. Non crystallized material will have same colour in both two windows.
… Crystallized material will have two different Colours to be viewed in these windows.

What is Luster Called in Gemstones?

Luster is referred to the light reflected from the surface of gems. In some gemstones, there is internal light reflections also such as facet stones with transparency. However internal reflection of light is not called luster. It is called brilliance. Luster is the way a mineral either shines or does not shine. Gems and minerals can have metallic and non metallic luster.

Various terminology are used for defining luster for a gemstones. Such as Adamant, Vitreous, Silky, Greasy etc. Normally luster can be defined in two categories:-

  1. Metallic (such as pyrite , hematite)
  2. Non Metallic (all gemstones)

One will get in gemstones most non metallic gemstones. Following are described brief description of each luster type:;-

  1. Adamantine
  2. Sub Adamantine
  3. Vitreous 
  4. Sub Vitreous 
  5. Greasy
  6. Silky
  7. Pearly
  8. Resinous
  9. Waxy
  10. Dull
Non-Metallic Luster Appearance
Adamantine Sparkly
Earthy Dull, clay-like
Pearly Pearl-like
Resinous Like resins, such as tree sap
Silky Soft-looking with long fibers
Vitreous Glassy

Adamantine

Adamantine is highest form of luster just like mirror. Its best example is Diamond.

Sub-Adamantine

Some gemstones has luster like diamond but they are not fully adamantine. However they are closed to adamatine. They somes in sub-adamantine category. Their examples are Cubic Zircon.

Vitreous

Some gemstones have glassy like shining on surface such as tourmaline, Quartz, topaz.  The majority of gems show a vitreous luster. Examples are beryl, quartz, topaz, ruby, emerald and others with refractive indices between 1.50 and 1.70.

Sub-Vitreous

In sub vitreous, the gemstones are included which have low reflection of light on surface such as flourite.

Greasy

Gems with greasy surface will have oil like or fat like. The example is serpentine. Another example is garnet or peridot. These luster are characteristic of those polished surfaces that are not perfectly flat but that approach flatness.

Silky

The gemstones with silky luster will be based on having some fibrous material such as Gypsum and Malachite. Another example is Ulexite or moonstone. They look like having texture of fabric. Polished tiger’s eye shows a vitreous luster on its surface, overlaying a fibrous body.

Pearly

Some gemstones have orient luster which is called Pearly. They show an iridescent play of colors on their surfaces.

Resinous

Amber are consisted of preserved prehistoric plant resign. This gemstone have resinous luster example in best. Sphalerite has also resinous luster. The gem materials that are soft and have low refractive indices, like Amber and Opal.

 

Waxy

Some gemstones looks like having kind of a wax on surface. Best example is turquoise and Opals

Dull

As this is shown by name that gemstone with having dull luster will be reflect very very low light. Dull will have fine grain like surface.

Metallic

Some gemstones or minerals have metallic matters in them. Such as Pyrite and Hematite. Though

they are not used as gemstones in general however in some cases, they are.

How to Photograph Gemstones and Minerals?

Photography of gemstones is an art . They have highly reflective surfaces that can reflect light back into the camera lens overexposing the facet and frequently burn into adjacent facets as well. Gemstones also have internal brilliance that act in much the same way as the surface reflections.  They always need specific light and background to make clear the quality of gemstones in such a way that a photo of gemstone can speak all its characteristics. Though it is not difficult to photograph the gemstone. The only method needed is how to deal with gemstones.

List of a Few Equipment you need before photographing a gem?

  • Camera or Mobile Phone – Camera should be capable of shooting images of 16 mega pixels or above.- In Case of Mobile, samsung zoom is good. iphone takes original colour photos. Main thing is needed to focus the object and light.
  • Macro Lens – 60-80 mm macro lens capable of 1:1 magnification enables you to focus on the object from 5 inches distance or more.
  • Tripod – Tripod will help you in movement. It will finish the movement of camera while taking photos.
  • Lighting and setting (A Light box will help )
  • wireless shutter

Camera Macro Lens

Cheap and Easy way to make Light Box

Setting before Beginning

  1. Wash your gemstones so that macro setting of camera may not capture the dust and scratches
  2. Stabilize the Tripod. If it is not available, set your camera in fixed position or set mobile phone
  3. For Mobile devices, use wireless shutter. Touching your mobile screen will not be able to focus on the object.
  4. Clip-on macro lenses are available for smartphones from a variety of manufacturers for Apple and Android devices. ōlloclip macro is a good one and is portable.

Choose your Background Setting

While coshing background, it is very important to crate a dramatic set up. If you want accuracy or focus on the gemstone or mineral, one will chose neutral coloured background. Background depends on gemstone colour or gemstone type. Some mineral specimens loks more clear in natural background. Some faceted stone looks more clear in plain background.  There can be two types of background settings:;-

  1. You can make a light tent out of almost anything that’s translucent.
  2. White Background 
  3. Black Ground
  4. Mix background 

You need to figure out which background is reflecting on stone much better than the other. Quartz will need black back ground. Blue will need white background. Aquamarine with more blue will need both types.

Lightning

Lightning plays important roles in the photography. In gems world, lightning has key role. Lighting determines the appearance of the gem–how the facets reflect the light–and the color of the gem, via the spectrum of the lighting.  Gems are of different colors which are reflected in different light setup. Some of the them are described here below:-

  1. White LED Day Light 
  2. Reflected Light 
  3. Yellow LED Light 
  4. Sunlight before afternoon 
  5. Sunlight 2 hours before sun set 
  6. Dont use Flash in most of the time especially when you are working with close up shots. 
  7. Adjust the white balance on your camera

Sometime, you have to take photos n white LED light and sometimes yellow light. Some times, you have to mix both. Sunlight in the morning till 1 hour before afternoon is the best time for taking photos. But its not necessary you get the best background always if one is living in cities metropolitan life. So you have to set up artificial day light.

Here are some tips over lightning:-

  • Try to use single colour light
  • Throw direct light on some gemstones like cut or those which has some affects like cat’s eye.

  • Some gems will demand indirect light. For this, light with white parachute can help. Throw light from outside parachute to inside which will throw the reflections.

  • A mix of direct and indirect or diffused lightning can make the picture beautiful by adding shining or dispersion to a transparent gem.

  • A high polished gems can be problematic with some direct lights. So that case, indirect lightning will help.
  • You can take picture in sunny day in a shadow because shadow will be reflecting the light.
  • Faceted stone would not work under sunny day as sunlight will sparkle the facet and you would not be able to catch the individual facet.

Day Light Bulb

Positioning of the gemstones

Positioning of the gemstone plays a vital role int he picture. The angle of gemstones matters a lot in final photo.

  1. If the stone is rough faceted grade like tourmaline to make clear xis view clear, its top surface has to put before light so light reflection can be seen easily.
  2. For stones like which have stars, they should be positioned in such way that through light, the star should appear in center.

 

Adjusting Focus

Focus is the main factor which plays a role in taking fine picture. In fact, this is the weak point. You have to select the point on stone to focus your camera. People need to know the maximum focal length of their lens. Pictures end up out of focus because they’ve gotten closer than the limit of the lens. They need to know where things go out of focus. If it’s too close, they need to use a different camera or a zoom mechanism.

 

If one will loss the focus area of mineral specimen, you are going to be out of giving unique detail of the specimen. It is better to take test photos for reviews.

Distance

  • One needs to be able to focus on an object from about 6 mm in diameter on up.
  • Optically the rule is that the longer the focal length of the lens the narrower the angle of view; hence the greater the working distance.
  • 105 mm lens is the best one as it will give opportunity to light to throw all around. Minimum length or distance would not allow throw of light. However with adjustment, 60 mm to onward works well.

Composition and Editing of Photo

After taking photos, editing is the final stage. It needs following activities :-

  1. Transfer the image in computer
  2. Cropping
  3. Colour balance
  4. Adobe Photoshop
  5. Different Aps in mobiles

Make sure that picture quality may not loss during editing.

Here is a tip to get macro photos from phone