CORUNDUM. ANISOTROPIC. UNIAXIAL. HEXAGONAL. COMPOSITION: Al_{2}O_{3}. _c_ = a. ▄Usual Appearance in Sections▄: Pyramidal or prismatic crystals, grains or basal plates. Zonal structure or twinning may be noticed. Colorless or with patches of blue. Index of refraction high (_n′_ = 1.766, α = 1.760, γ = 1.769), hence _relief_ well marked and surface very rough. Rhombohedral cleavage may show in larger individuals. Pleochroism only marked when color is deep. ▄Crossed Nicols▄: Double refraction weak (γ − α = 0.009), like quartz. Interference colors middle 1st order, white to yellow. _Extinction_ parallel in elongated sections. Optical anomalies very rarely noticed in microscopic individuals. In _convergent light_ basal sections show a rather indistinct cross; optical character (−). ▄Distinguished from▄: (_a_) APATITE and VESUVIANITE by brighter interference colors. (_b_) TOURMALINE (light colored) by not having such strong absorption. (_c_) CYANITE by uniaxial character. Corundum may need to be isolated from the rock in order to be determined with certainty. REMARKS: found in contact metamorphic rocks, eruptive rocks, granular limestones, etc. It is insoluble in hydrochloric acid. When rock sections are ground with emery, care must be taken not to confuse grains of emery with corundum in the rock. H., 9. Sp. gr., 3.9 to 4. QUARTZ. ANISOTROPIC. UNIAXIAL. HEXAGONAL. COMPOSITION: SiO_{2}. _ć_ = c. ▄Usual Appearance in Sections:▄ Allotriomorphic in the granitoid rocks, when apparently the last mineral to form, Fig. 5. More or less chemically corroded pyramidal crystals (with cross-sections six-sided or rhombic with an angle of about 100°) in the porphyritic rocks. Rounded or angular grains in the “clastic” rocks; granular mosaic in crystalline schists and contact rocks, very rarely in distinct crystals in any rocks. May at times be mutually interpenetrated with an acid feldspar (the areas of quartz and feldspar extinguishing as entire crystals), producing “micro-pegmatitic” structure, Fig. 67. Finally may appear as pseudomorphs after other minerals, but may then consist of some of the other forms of silica. _Color._—Colorless, although by reflected light it may appear colored or cloudy if it contain many inclusions. _Index of Refraction._—_n′_ = 1.547 (α = 1.544, γ = 1.553) hence no _relief_ and surface smooth. _Cleavage._.—Rarely noticed, an important fact in determining quartz. Quartz breaks irregularly.