Fracture, referring to any surface except that of a cleavage fall, may be uneven, conchoidal (shell-like), hackly (rough), etc.
Tenacity refers to such qualities as brittle, sectile, malleable, flexible, or elastic.
Hardness is represented by the difficulty with which a smooth surface is scratched. The scale in general ore was devised by Mohs. It is:
- Talc. Scratched by the finger nail.
- Gypsum. Ditto, but with more difficulty. Will not scratch a copper coin.
- Calcite. Scratched by a copper coin.
- Fluorite. Is not scratched by a copper coin and does not scratch glass.
- Apatite. Scratches glass, but with difficulty. Is readily scratched by a knife.
- Feldspar. Scratches glass with ease. Is difficult to scratch by knife.
- Quartz. Cannot be scratched by a knife and readily scratches glass.
- Topaz. Harder.
- Corundum. Harder.
- Diamond. Scratches any other substance.
Hardness may be intermediate. For instance, any mineral that scratched quartz and is soft enough to be scratched by topaz, in turn would be rated at 7.5.
Specific Gravity. This is the density of mineral and other substances compared with that of water. It is particularly valuable in determining heavy metals.
To find the specific gravity of any solid body divide its weight in air by the loss of weight in water, at a temperature as near 60 degrees F. as possible, and the quotient will equal the specific gravity. In the case of gases, such as nitrogen, oxygen, etc., hydrogen is taken as the unit.
Luster. There are seven kinds of luster, viz: Metallic, the luster of metals; adamantine, that of the diamond; vitreous, of broken glass; resinous, of the yellow resins; greasy; pearly; silky. There are five degrees of intensity of luster recognized, viz: Splendent; shining; glistening; glimmering; dull.
Color and Streak. The streak is the color of the powder of the mineral when rubbed on unglazed porcelain, or scratched with a knife.
Transparency. Minerals may be transparent, sub-transparent, translucent, sub-translucent, opaque.