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alumina
and thjs compound in its crystallized condition is known as corundum.
Only a few common elements go to form gem-stones. Diamond is the only
gem-stone made up of a single element. All the others are chemical
compounds mostly containing oxygen, except one which is a compound of
fluorine. The elements that are common in gem-stones are silicon,
aluminium, iron and magnesia. Besides, other elements may occur in
traces or in small amounts.
The
elements can be represented by their chemical symbols and atomic
weights and their combinations by chemical formula. Two atoms of
aluminium and three atoms of oxygen go to form alumina (corundum or
ruby) which is written down as A1208 (a molecule).
For
a scientific classification of gem-stones, a knowledge of the chemical
composition is essential. Equally essential is the crystalline
structure. All other characters depend on them. Chemical tests are not
very useful for gem-stones as they will require the destruction of the
stones. Hence for purposes of determination, physical tests which do
not harm the stones are the most important.
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