Flint has II. = 7, and only about a dozen minerals, including
the precious stones or gems, are harder. Precious stones have
latterly been divided and arranged according to their hardness,
in the following three classes • 1. HARD GEMS ; OF THOSE HARDER
THAN QUARTZ. 2. SEMI-HARD JEWELS. Book Crystal. Opal. Carnelian,
and other Obsidian, similar ones. Turquoise. 3. SOFT PRECIOUS
STONES. Those softer than Fluor-spar ; Malachite, Amber, and
Jet. • Closely allied to hardness is the tenacity of minerals,
of which the following varieties have been distinguished : A
mineral is said to be brittle when, as in quartz, on attempting
to cut it with a knife, it emits a grating noise, and the
particles fly away in the form of dust. It is sectile or mild
when, as in galena and some varieties of mica, on cutting, the
particles lose their connection in a considerable degree ; but
this takes place without noise, and they do not fly off, but
remain on the knife. And' a mineral is said to be soft or
'ductile when, like native gold