The principal rock minerals which make up the bulk of rock formations of gold deposits are basically very few. It is important to recognize quartz, feldspar, mica, hornblende and augite. It is the different ways in which these gangue mineral are combined that distinguish the igneous rocks from each other; and it is the predominance of one or the other in the secondary rocks which imparts to these rocks their peculiar characters. Granite consists of quartz, feldspar and mica; syenite, of crystallized feldspar and hornblende; and basalt of feldspar and augite, with chrysolite or olivine, so that with specimens of these rocks or even in some cases with granite and basalt only, the gold prospector must be ready to study the characters by which the minerals may be recognized.
Quartz is the glassy portion of granite which cannot be scratched with knife. It crystallizes in the well form, the separate crystals being always prisms, terminating in a rather blunt pyramid with six sides or faces; so that the description will be as follow: crystallizes in six-sided prisms with six-pyramid for a termination; color sometimes tinged with pale smoky or rose color, sometimes violet as in the ameththyst, usually colorless and transparent or milky white; luster, vitreous; harness, cannot be scratched with a knife, and itself scratches feldspar; no acted on sulphuric, nitric, or hydrochloric acid.
Feldspar is the white portion of granite which can be scratched with a knife. There are several varieties of feldspar, distinguished by their having either soda, potash or lime as one of their constituents in addition to the silica and alumina which are the main compounds, but it is not easy to give differences which would be easily recognizable by the gold prospector without much experience. The crystallized forms are usually white, more or less inclined to be transparent or translucent in new fractures, but often weathering to a milky white. The feldspar in granites very in color from white through pink to dull red and often occur as seams of varying width running through the body of the rock. These seams are inclined to break into squarish fragments.
Mica is the minerals which in thin plates is often wrongly called isinglass (which is fish glue).in color it varies from colorless or white to black, through various shades of gray, brown, yellow, green and violet. The commonest colors being white, yellow, dark brown, dark green and black. While sometimes found in crystals of large size, they are usually quite small. The crystals are flat, six-sided and invariable split into extremely thin plates parallel to the base of the crystal. When broken across the crystal, as is often the case in a rock fracture, the characteristic six-sided form may not be visible, but the thin plates separate easily into a brushy edge. These plates are elastic and can be bent considerable without breaking, by which character the white varieties of the mineral can be distinguished from crystallized varieties of gypsum, which are also white and transparent.
Hornblende occurs usually in small crystals, usually black or greenish black. It is harder than mica and does not split into thin layers, which can be determined by the use of a knife. It crystallizes in little squares and sometimes in six-sided crystals, which may be easily distinguished from mica by having two of the opposite sides much wider than the others. If often occurs in greenish or blackish masses with a fibrous or radiated structure, sometimes forming a rock almost by itself and grades down asbestos, which has practically the same composition. Augite is similar in appearance to hornblende except that the crystals in cross section show eight sides, and is much less important than hornblende to the gold prospector of rock composition, until he has made some progress. We have thus two usually pale or white minerals, and three dark brown, greenish or black minerals to deal with, and a very small amount of practice will enable any prospector to pick them out easily. The knife will indicate the difference between quartz and feldspar, but mica, hornblende and Augite are easily scratched with the knife giving colorless streak and the difference between the rocks minerals must be determined by the shape of the crystals.