Lustre

Lustre, a somewhat complex character of minerals, not easy of definition, consisting in the quality and intensity of the light reflected from their surfaces. The kind or quality of lustre depends partly upon structure, partly upon transparency, and largely upon refractive power; the degree of lustre, upon the amount of reflection.

Minerals with perfectly smooth faces are either metallic, adamantine, resinous, or vitreous. If opaque and with an index of refraction (q.v.) above 2-5, they are metallic (1), as in most native metals and their sulphides, such as galena and pyrites. If with an index between 25 and 1-9, they are adamantine (2), whether transparent, as diamond, or opaque, as blend. If translucent only, with an index between 1-9 and D7, they are resinous (3), as in garnet. An indefinable variation of this, resembling wax, seen in some opal and in hornsilver, is termed waxy (4). If transparent and with an index between 1 -8 and 1-3, as in ice, fluor, quartz, rock-salt, calcite, and sapphire, the mineral is termed vitreous (5). Numerous lamellae within a translucent mineral, whether well-developed cleavage-planes, as in mica and selenite, or the result of incipient decomposition, produce pearly (6) cleavage. Fibrous structure, such as that of asbestos and satin-spar, produces silky (7) lustre. In degree, lustre is either (1) splendent, where a well-defined image is reflected, as in specular haematite; (2) shining, where only ill-defined images can be reflected, as in celestine or baryte; (3) glistening, where there is a general surface reflection but no recognisable image, as in mica; (4) glimmering, when there is only a faint reflection from scattered points on the surface, as in flint; or lastly (5) dull, when there is scarcely any white light reflected from the surface - a character mainly exemplified by ochres and other earthy minerals. Unlike faces of the same crystal often differ in lustre, pearly and silky lustre especially being commonly confined to particular faces, those parallel to the cleavage.