CLASS I. STONES. 
ORDER I. EARTHY STONES. 
/. HARD : those 'which scratch Glass. 
49. OF GEMS IN GENERAL. 
GEMS, or precious stones, as they are frequently 
called, are, for the most part, transparent, and have a 
vitreous or glassy appearance. Their different colours 
are occasioned by metallic oxides (21) of various 
kinds, with which they are impregnated. Some writers 
have classed them by their colours, but this is a very 
uncertain mode, as different gems have not unfrequently 
the same colour ; and, in many cases, the same gems 
are of different colours. The usual distinction of gems 
into Oriental and Occidental is also liable to error, as 
the best gems, from whatever part of the world they 
are brought, are always called Oriental. The most 
estimable of all the kinds are the diamond (50), ruby 
(54), emerald (67), and sapphire (53); and stones a 
grain in weight, and equal in quality, are valued in the 
following proportions, at 8l. per carat for diamonds, 4/. 
for rubies, and 3/. for each of the others. The amethyst 
(79), topaz (61), and aqua-marine (61)> are considered 
of nearly equal value with each other ; and the garnet 
(70) is the cheapest of precious stones. 
