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 or lead, it can be cut into slices with a knife, extended under
the hammer, and drawn into wire'. From tenacity it is usual to
distinguish frangi-bility, or the resistance which minerals
oppose when we attempt to break them into pieces or fragments. This
property must not be confounded with, hardness. Quartz is hard, and
hornblende comparatively soft; yet the latter is