130 A POPULAR TREATISE ON GEMS.
in
the mineral kingdom. In the most transparent quartz crystals, traces of
alumina and iron oxide can be detected; the purest spinel contains a
small amount of silica, and the most brilliant diamond, consumed by the
solar rays, leaves some ash behind. Such non-essential mixtures must be
neglected, or each individual crystal would form a distinct mineral
species. The isomorphous elements introduce a wider range of varieties,
and render the limitation of species more difficult.' Carbonate of
lime, for instance, becomes mixed with carbonate of magnesia or of iron
in almost innumerable proportions; and the latter substances also with
the former. Where these mixtures are small in amount, variable in
different specimens, and do not greatly affect the form or physical
characters of the predominant element,
"
they may safely be neglected, and the mineral reckoned to that species
with which it most closely agrees. Where, however, the mixture is
greater, and the two substances are frequently found in definite
chemical proportions, these compounds must be considered as distinct
species, especially should they also show differences in form and
other external characters.
Amorphous
minerals with definite composition must also be considered as true
species. But when they show no definite conrposition, as in many
substances classed as clays and ochres, they cannot be accounted true
mineral species, and properly ought not to be included in a treatise on
mineralogy. Some of them, however, from their import-
'
ance in the arts, others from other circumstances, have received
distinct names and a kind of prescriptive right to a place in
mineralogical works, from which they can now scarcely be banished. Many
of them are properly rocks, or indefinite combinations of two or more
minerals', others are the mere products of the decomposition of such
bodies. Their number is of course indefinite, and their introduction
