Amber fossil resin 2.0-2.5 1.05-1.10 about 1.54 brown, yellow Augite CaMgSi₂O₆ 5.0-6.0 3.2-3.6 1.60-1.71 greenish brown, black Beryl Be₃Al₂(SiO)₆ 7.5-8.0 2.63-2.80 1.56-1.60 pale blue, colorless, greenish Celestite SrSO₄ 3.0-3.5 3.95-3.98 1.62-1.63 colorless, blue Epidote HCa₂(Al, 6.0-7.0 3.25-3.50 1.72-1.77 yellowish Fe)₃Si₃O₁₃ green, brownish green Fluorite CaF₂ 4.0 3.0-3.25 1.434 colorless, violet, yellow, green Garnet Fe₃Al₂(SiO₄)₃ about 7.5 4.25 about 1.83 red, deep (Almandite) red, brownish red Labradorite NaAlSi₃O₈ 50% to 6.0-6.5 about 2.6 about 1.56 yellowish, 30% CaAlSi₃O₈ 50% grayish to 70% Microcline KAlSi₃O₈ 6.0-6.5 2.54-2.57 1.52-1.53 pink, red, bluish, greenish, white Obsidian volcanic glass 5.0-5.5 2.3-2.5 1.45-1.53 dark gray, black, brownish Opal SiO₂·nH₂O 5.5-6.5 1.9-2.3 1.43 white, pink, bluish, brown, gray Quartz SiO₂ 7.0 2.65-2.66 1.544-1.553 colorless, (Crystalline) violet, yellow, brown Tektite natural glass 5-6 2.33-2.44 1.48-1.52 dark brown, (Bediasite) greenish brown