Certain minerals are dense such as bitumen, stones, and metals while others are tenuous such as the spodos found in mines, natural lime, melan-teria, and
a black mineral which resembles the down from plants and from which we
obtain quicksilver (metacinnabarite). We apply the term dense to
minerals which, because of strong coherence, occur in large units and
apply the term tenuous to minerals which occur in small units because
of the lack of coherence. Thus hard and tough minerals, since they
occur in large units, are dense and soft and easily crushed minerals
which occur in small units are tenuous. Impressions can be made in
certain minerals of the latter class such as unctuous, incoherent, soft
or moistened earths, bitumen, lead. Stones and most minerals will not
take impressions. Some minerals soft enough to take impressions of the
hand or blows of a hammer may be worked into various shapes while other
minerals which are harder may be cast. Copper can be cast as well as
worked by hammering. Some minerals, too hard to take impressions, are
soft enough to be scratched by iron, for example, marbles, almost all
rocks and many stones known by special names. Other minerals are too
hard to be scratched such as flint and almost all transparent gems.
Minerals soft enough to be scratched by iron may be engraved and even
turned in a lathe, for example the Zeblician marble of Misena and the
Comensis stone of Italy. Goblets are turned from the former and,
according to Pliny, cooking utensils were turned from the latter.
Venetian tofus can be cut with iron and rocks and marbles can
be split with wedges. Some of the minerals which cannot be scratched
with iron are very brittle such as flint while others such as the knots
found in schistos and basaltes can be broken only with great effort. Some minerals, for example the diamond, are not affected by a blow.
Some gems are scratched by a file such as topazius (chrysolite) while others are not, such as lapis-lazuli and carbunculus. All gems can be engraved with emery except the diamond which can only be scratched by its own fragments.
Certain minerals can be compressed such as native spodos and
a black mineral similar to the down of plants and which is sometimes
argentiferous, sometimes hydrargyriferous (metacinnabarite). Common
stones, gems, rocks, marbles, and metals cannot be compressed. Porous,
unctuous, soft earths can be compressed and when compressed, remain so.
Some can be drawn out, i.e., protuberences of unctuous earths which
have been moistened with water, as well as bitumen can be drawn out
into a long thin body because these have the essence of movement.
Earths which are hard, meager or dry cannot be drawn out. Gold, silver,
and copper can be drawn as well as stamped. None of the stones can be
drawn. Bitumen can be drawn and compressed. Some minerals are
cleavable, for example selenite and talc, for the openings in these
minerals have been extended along the length, not the width. Some occur
similar to hair such as native silver and alum. A great many do not
have a cleavage such as stones and metals. Some minerals burn readily,
sulphur, bitumen, and jet, while metals,