out
a lengthened dissertation on the goldsmith's art, which would have
caused us to deviate from the principal subject. Besides, this subject
has already-been treated by me in a short work entitled Dell' Oreficeria antica, and perhaps will be again considered in a more voluminous work.
The
genera of the third class, to which belong almost all the substances
called gems, are, as stated in the introduction, twenty-four; but we
need not name them all, those being sufficient which include every
substance partaking of the nature and name of,precious stones.
All gems, then, are arranged under the following genera and orders of the third mineralogical class :
The
1st. Non-metallic oxides. 6th. Aluminates. 7th. Aluminous silicates.
8th. Non-aluminous silicates. 9th. Silicates mixed with other
components.
To the first genus, of non-metallic oxides, belong the species—
1st. Of corundum, some of whose varieties are :
