Physical Property of Gem Stones
In physical properties we will study following things are hardness, cleavage, toughness, fracture and specific gravity etc.
Hard is the degree or ability of a solid or material to defend the integrity of their surface against cause of division or abrasion. The hardness depends upon the “cohesion or the chemical and electrical forces of the atomic structure so called the atomic bonds.
Assessment of the hardness of a gem
We can judge or access the hardness of a stone with a hand lens. Flatness of the surface & sharpness of the edges show that the material is hard. If a material is soft on a mounted stone we can see lot of scratches on the surface of the facets and abraded edges, like in glass imitation but not in case of diamond and corundum.
Determination of hardness test
In 1822 after intensive study and analysis Friedrick Moh chose ten minerals in order( 1 to 10) of hardness to serve as standards of comparison. It was given name Moh’s scale. This test shows that gems of lower number can be scratched by gem of higher number but gem of lower number can’t scratch the gem of higher number. The gem of same number can scratch each other.
Moh’s scale is as:-
It can be scratched by finger nail with low pressure.
It can be scratched by finger nail with greater pressure.
It can’t be scratched with finger nail but can be scratched with knife.
It can be scratched by finger nail with low pressure.
A fine scratch can be made with considerable pressure with knife.
It cannot be scratched with knife but can be scratched by an edge of steel file.
It can’t be scratched by steel file but can scratched by Topaz N:8 with pressure, use lens to see scratch.
It can be scratched by Corundum N:9.
It can be scratched by diamond N:10 among natural stances artificial silicon carbide compound called Carborundum.
It can only be scratched by other diamond.
The greatest single defect in Moh’s scale is, it doesn’t mean that diamond is ten time harder than talc or the difference of hardness between Quartz and Topaz is same as that of Corundum and Diamond.
Importance of hardness in gemstones
The main importance of the hardness lies in the durability of the gem & a display of brilliant lustre. Hardness enable the stone to holds it’s lustre & polish for a long period.
It is a plane or direction of weak molecular bonding by which it can split along definite smooth surfaces. Cleavage planes are always parallel to the face which already exist on the crystal. Gemstones like diamond and topaz have perfect cleavage. Mica has such a perfect cleavage that it can be splits with fingers into sheets as thins as paper. Cleavage is a crystalline property.
Types of Cleavage
Cleavage is of three types
In this type of Cleavage the produced surfaces are very Smooth.
E.g. Topaz, Diamond.
Here the produced surfaces are smooth but slightly granular.
Whenever produced, surfaces are rough with irregularities. We call it imperfect cleavage.
Characteristics of Cleavage
- This property helps in the identification of the stone.
- Cleavage helps in Diamond cutting. Octahedral cleavage of Diamond is used in trimming of stone of awkward shape, before the grinding process begins.
- Cleavage also helps in polishing process. Stones takes good polish if they are not polished along cleavage direction.
If a stone is cleaved in wrong direction this may cause gemstone to break or fracture and this is one of the characteristic of the gem material in identification. This property is possessed by all solid crystalline and amorphous. The nature of the surface is rough and irregular.
Difference Between Fracture & Cleavage
- Any irregular break or crack in the stone other than the cleavage and parting direction.
- Fracture can take place in any arbitrary direction.
- Due to fracture produced, surface is irregular.
- It may at any place.
- Whereas here stone can split and give two smooth surface if there is cleavage found.
- It has definite directional property.
- In cleavage, surface is comparatively smoother.
It always occurs parallel to some
The density of a material is defined as its mass per unit volume. It is the ratio between mass of the stone and the mass of the identical volume of pure water at 40° C. Specific gravity depends upon the atomic wt. and compactness of the atoms in the structure.
Determination of Specific Gravity
There are different methods and ways to determine the specific gravity of the stone but most often these two are used as:
Here we first weighed the stone in air i.e. (Wa). Then same stone weighed in water i.e. (Ww). So the Wa will be more than Ww becase the stone is buoyed up by a force equivalent to the Wt. of water displaced. In this way we can find the specific gravity of the stone.
We have an another method which is quick
Heavy Liquid Method
This is more common method due to its quickness. Actually heavy liquids are the liquids which posses much higher sp. gravity than that of water & even than stones.
So concept is simple, if the sp. Gravity of the stone will higher than the liquid, it will sink in the liquid & if it is lower than of liquid the stone will float in the liquid. Where as sp. gravities of both liquid & stone are equal than stone will remain stationary in the liquid.
For this type of test the requirements of the liquid are as:
- The liquid should be non-reactive
- It should be transparent.
- It should be easily mixable with other solutions.
- It must be easily available and should not be too expensive
- It shouldn’t be too poisonous.
- It should not be highly inflammable.
- It must have high sp. gravity.