The percentage composition of peridot, though the ferrous oxide may be more or less replaced by magnesia, is approximately :
Silica.....41 | Ferrous oxide ... 9
Magnesia . . . .50
Manganese, nickel, and lime have been found in small quantities in some olivines.
The
term olivine is now wrongly applied by dealers in precious stones and
by jewellers to the green garnets or demantoid from Bobrovka ; these,
for a time at least, were called, with equal incorrectness, Uralian
emeralds. A considerable quantity of pale green cut gemstones has been
sold in London as peridot, but on examination proved not to be that
variety of chrysolite. They were softer than peridot, and, though more
glittering in lustre, were of poorer colour. They were not dichroic and
showed no sign of crystalline structure. On analysis they were found to
contain much more silica and much less magnesia than peridot, while
alumina and soda were present in distinct quantities ; if the material
had not been known to be a natural product, these specimens would have
been called green glass. Their composition varies widely, and so does
their density, the latter ranging from 2'36 to 2"63. The mineral which
occurs in several Bohemian and Moravian localities is known as
water-chrysolite, psendo chrysolite, moldavite and bouteillenstein. Various
opinions are held as to its origin, some mineralogists even going so
far as to state it to be an artificial glass from the site of early
glass factories. It has also been conjectured that it may be of
meteoric origin. Some glass has undoubtedly been sold as moldavite, but
the genuine material differs in intimate structure, in fusing point and
in chemical composition from any kind of glass. It is not identical
with obsidian, while its occasional occurrence as rolled pebbles in the
gem-gravels of Ceylon strengthens the view that there is a moldavite
which is a natural product.
