is 5.5; specific gravity, about 2 to 4; lustre, vitreous; translucent to opaque.
LAVA
can hardly be classed as a semi-precious stone, but it is and has been
quite extensively utilised in jewelry, chiefly on account of
sentimental association with, and as souvenirs of, volcanoes. Lava is
the fusion of various mineral substances due to the heat and force of
eruptions from the interior of the earth; it varies in structure and
constituents, but the surface lava is usually massive with vesicular or
porous marks; fracture, splintery and conchoidal; lustre, dull or
glistening; it is opaque and of various colours and shades. Lava
frequently contains crystals—feldspar, lenate, hornblende, garnet, and
other minerals. Vesuvian lava of a blue tint resembles transparent
enamel, and is mounted in brooches and rings; cameos and intaglios are
sometimes cut on it.
MAGNETITE, or lodestone, possessing polarity, is used for charms, because of the mystical properties attributed to it.
MALACHITE
is carbonate of copper of a bright green colour. When this copper ore
occurs in conjunction with azurite, the companion minerals are cut
together, with a pleasing effect.
OBSIDIAN is compact volcanic glass, and is
