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

 

 

 

 

 

 

 

 

 

 

 

 

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