Color: oxide Mohs’ hardness: 9 Transparency: Opaque, translucent, transparent Luster: Vitreous to adamantine Fracture: Small, conchoidal, uneven, splintery, brittle Crystal system: Hexagonal (trigonal); hexagonal prisms or tablets, rhombohedrons Cleavage: None Specific gravity: 3.97-4.05 Refractive index: 1.766-1.774 Dispersion: 0.018 Pleochroism: Strong; yellow-red, deep ruby-red Fluorescence: Strong; ruby-red Occurrence: Worldwide Astrology: July birthstone, also connected with Cancer, [...]

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Color: Colorless, yellow, pale sheen; rainbow moonstone has blue sheen Chemical composition: K(AlSi3O8) potassium aluminum silicate Mohs’ hardness: 6-6 ½ Transparency: Turbid, transparent Luster: Vitreous Fracture: Uneven, conchoidal Crystal system: Monoclinic; prismatic Cleavage: Perfect Specific gravity: 2.56-2.62 Refractive index: 1.522-1.530 Dispersion: 0.012 Pleochroism: None Fluorescence: Weak; bluish, orange Occurrence: Australia, Brazil, India, Malagasy Republic, Mexico, [...]

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Color: Sapphire-blue, amethyst, violet Chemical composition: Ca2Al3 (O/OH/SiO4/SiO2) calcium aluminium silicate Mohs’ hardness: 6 ½-7 Transparency: Transparent Luster: Vitreous Fracture: Uneven, brittle Crystal system: Orthorhombic, multi-faceted prisms, mostly striated Cleavage: Perfect Specific gravity: 3.35 Refractive index: 1.691-1.700 Dispersion: 0.030 Pleochroism: Very strong; violet, blue, brown Fluorescence: None Occurrence: East Africa (Tanzania) Astrology: recently designated a [...]

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Color: Colorless, red, orange, yellow, green, blue, violet, nearly black Chemical composition: CaF2 calcium fluoride Mohs’ hardness: 4 Transparency: Transparent, translucent Luster: Vitreous Fracture: Even to conchoidal, brittle Crystal system: Isometric; cubes, octahedra Cleavage: Perfect Specific gravity: 3.18 Refractive index: 1.434 Dispersion: 0.007 Pleochroism: None Fluorescence: Usually strong; blue-violet Occurrence: Canada, China, England, France, Germany, [...]

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Fossil coral was once the skeleton of a coral polyps whose organic structure has been transformed to stone. After the coral died, the skeleton became saturated with water that contained dissolved silica, the same mineral that is the principal component of quartz, chalcedony, agate, and jasper. In this case, the silica replaced the coral’s soft [...]

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