It
is thus somewhat harder than ordinary glass. Its specific gravity
ranges from 2.32 to 2.36. Unlike ordinary glass and obsidian it is
almost infusible before the blowpipe, and when fused remains perfectly
clear on cooling. It differs considerably in chemical composition from
ordinary glass, having as it does a higher percentage of silica,
considerable alumina, and a small percentage of alkalies. The
percentages of silica range between 88 per cent and 78 per cent; those
of alumina between 5 per cent and 13 per cent; and those of potash and
soda between 1 per and 2.5 per cent. The following is an analysis of a
dark green moldavite from Budweis: Silica, 77.75, alumina, 12.90, iron
protoxide, 2.60, lime, 3.05, magnesia, 0.22, potash, 2.58, soda, 0.26,
water, 0.10. In ordinary glass the percentage of silica is not much
above 50 per cent; there is almost no alumina, while lime and magnesia
amount to about 20 per cent, and potash and soda 20 per cent to 25 per
cent.
Glassy
pebbles similar to moldavite are found on the island of Billiton, near
Java. These are known as billitonite. They are also found in Borneo and
several parts of Australia. In these places they are believed to be of
volcanic origin if not meteoric.
Of
these different occurrences of moldavite only the Bohemian is so far
used to any extent in jewelry. Owing to the abundance of the material
the stones cut from it are not expensive, being valued at no more than
quartz or agate. Actual glass can easily be substituted for it with
little chance of detection.
