The molecular weight of a compound is the sum of the atomic weights of the atoms in the elements that form these compounds. The atomic weight of an element is the weighted average of the atomic masses of the different isotopes of an element. For example, naturally occurring carbon is a mixture of two isotopes, 12C (98.89%) and 13C (1.11 %). Individual carbon atoms therefore have a mass of either 12.000 or 13.03354 atomic mass units (amu). But the average mass of the different isotopes of carbon is 12.011 amu.
The ratio of the velocity of light in air to its velocity in the substance is known as the refractive index of that substance. Almost every gem stone has it's own unique specific refractive index (RI). Measuring of the refractive index is probably the most conclusive measurement available to a gemmologist to identity a gemstone.
The Moh's scale of hardness is used to compare the relative hardness of different gem materials. The values are as follows:
10 - Diamond (hardest)
9 - Corundum
8 - Topaz
7 - Quartz
6 - Feldspar
5 - Apatite
4 - Fluorite
3 - Calcite
2 - Gypsum
1 - Talc (softest)
Specific gravity is the ratio of the weight or mass of a given volume of a substance to that of another substance (usually water for solids and liquids) used as a standard. Specific gravity is purely a ratio and does not have units .
Water has a density of 1 gram/millilitre. The density of any substance is the weight of the material compared to an equal volume of water.
Crystals are divided in to seven systems, each corresponding to a specific set of angular, geometrical, and symmetrical specifications. Every gem group can be classified according to its crystallization. The study of crystallization is invaluable in the determination and identification of rough gem materials.