What characteristics must a substance have to be considered a mineral?

Certain minerals (e.g. Zircon, almandine and those containing cerium) when examined with a spectroscope by transmitted light exhibit characteristic absorption spectra. The colours of minerals may also be due to the interference of rays of white light at the surfaces of thin crevices or minute inclusions, either tabular or fibrous in form, in the mineral; for example, the play of colours of opal; the change of colours of labradorite; the bands of rainbow colours (Newton's rings) seen along cleavage cracks and irregular internal fractures (e.g. In quartz); the iridescent tarnish due to a superficial film of a decomposition product (e.g. " peacock copper ore "); or the bluish opalescence of moon-stone and cat's-eye. The true colour of a mineral is best revealed by its " streak," i.e.

The colour of its powder. This is obtained by scratching the mineral, or by crushing a fragment of it on a sheet of white paper, or rubbing it upon unglazed porcelain. The streak of allochromatic minerals is white, while that of idiochromatic minerals is coloured and is often of determinative value.

Ores of iron may, for example, generally be distinguished by their streaks: that of magnetite being black; haematite, blood-red; limonite, yellow; and chalybite, white. The streak of a mineral may be either shining (e.g. Argentite) or dull. Another character depending on light is that of lustre, which is often very characteristic in certain minerals, though it may be considerably modified by the state of aggregation.

For example, the usual adamantine lustre of diamond is not exhibited by the compact aggregate known as carbonado; while earthy masses of any mineral will be devoid of lustre. Descriptive terms applied to the kinds of lustre are: metallic (e.g. Pyrites), adamantine (diamond), vitreous (quartz), resinous (pyromorphite), greasy (elaeolite), waxy (chalcedony), pearly (talc, heulandite and other minerals with a perfect cleavage), silky (satin-spar), &c. The degrees of intensity of lustre are described as splendent, shining, glistening, glimmering and dull, and depend usually on the smoothness of the crystal-faces.

The phenomena of phosphorescence, fluorescence and radio-activity (q.v.) are strikingly exhibited by some minerals. (See Fluor-Spar, Diamond, &c.) b. Magnetic, Electrical and Thermal Characters.