Physical and Optical properties of Gemstones.
Hardness
Hardness is related to the atomic bonding of a substance i.e. loose bonding between atoms generally gives rise to lower hardness and vice versa. Diamond and Graphite , two molymorphs of carbon have a hardness of 10 and 2 respectively. Moh's Scale
This is a universally used scale of hardness devised by German Mineralogist Friedrich Mohs(1773-1839). This scale is based on the principle that harder material will scratch a softer material. It is not a linear scale. Ten standard minerals used in Moh's scale are easily obtainable materials.
| Material | Hardness |
|---|---|
| Talc | 1 |
| Gypsum | 2 |
| Calcite | 3 |
| Flourite | 4 |
| Apatite | 5 |
| Orthoclase Feldspar | 6 |
| Quartz | 7 |
| Topaz | 8 |
| Corundum | 9 |
| Diamond | 10 |
Specific Gravity
Specific Gravity of a substance is defined as the ratio of weight of a body compared with the weight of an equal voluem of pure water. It can be measured with either hydrostatic method or heavy liquid method.
Heavy liquid method
This method uses 3 or 4 liquids having specific gravity betwee 2.65 and 4.15. The gemstone under test is immersed in the liquid to determine its approximate specific gravity.
If the stone sinks in the liquid , then specific gravity of the stone is greater than that of the liquid. If the stone floats at the surface of the liquid, The S.G. of the stone is less than that of the liquid. If the stone is freely suspended with in the liquid then the S..G of the stone is equal to that of the liquid.
Specific Gravity of Benzyl Benzoate is 1.17, carbon tetrachloride 1.59,bromoform 2.88,Methylene Iodide 3.33, Clerici Solution 4.00.
Refractive Index
When light passes through a substance, it bends.This is expressed as the ratio of velocity of light in the air to its velocity in the substance.This is measured with the help of a refractometer.This is one of the important property for differentiating gemstones.
some gemstones have single refraction and some have double refraction. This is determined with the help of an instrument known as Polarimeter and it relates to axis of gemstones.Gemstones can be Uniaxial or Biaxial.
Magnification
This is viewing the gemstone through a microscope or for most purposes a 10x lens suffice.This picture gives lot of information about gemstone.By naked eye it is not possible to make any conclusion about a gemstone.
