Quartz is composed of the two elements occurring the most abundantly in the earth's crust, silicon and oxygen, both non-metals. As already indicated, the most common representative of the mineral substance is the sand of the soil. The sand grains are generally so eroded by the atmosphere and surface waters as to show little of the true quartz structure. A typical specimen of quartz, commonly known as "rock crystal," clear, transparent and approximately perfect in form, is not difficult to obtain for study. If not occurring in the particular locality it may be obtained from a dealer in minerals at slight expense. As studied by means of the rock crystal, quartz is remarkable for its transparency, its regular crystal form, and its great degree of hardness. Its transparency is such that printing may be read through the crystal. Its crystalline form affords an unfailing means to the mineralogist of recognizing the substance as quartz. If our specimen be large and perfect, we note that it is bounded by planes in such manner that we have a hexagonal prism terminated at either end by a hexagonal pyramid. With convenient apparatus for measurement, we learn the all-important fact to the mineralogist, that the angle between any prism face and an adjacent pyramid face is 141° 47´. The mineralogist obtains his accurate measurements by means of an instrument known as the goniometer. We may obtain cruder results by bending a readily flexible wire over the two faces, perpendicular to the edge of their intersection, until it is tight against either face. Then placing one arm of the bent wire along the base of a protractor, the point of flexure at the center of the base, the number of degrees between the two arms may be read, thus giving roughly the angle between the prism and pyramid faces of the quartz crystal. The great hardness of quartz is apparent in that it cannot be scratched with the point of a knife and that it will cut glass. Often clear parts of quartz crystals occur studding the surface of a rock structure, in the form known as a crystal aggregate. One property of quartz rock (any sandstone or quartzite) we must not fail to notice is the irregular fracture. This is recognized in the statement that quartz has no cleavage.