Chemistry: Titanium dioxide (polymorphous with anatase and brookite)
Crystal system: Tetragonal; vertically striated prisms capped with pyramids; geniculate twins; sometimes repeated twinning causes closed rings.
Color: Rarely transparent; red, brown, black; too dark for gem.
Hardness: 6 – 6.5
Cleavage: Distinct: 1 direction; fracture: brittle, conchoidal to uneven.
Specific gravity: 4.2 – 4.3
Refractive index: 2.62 – 2.90; Uniaxial positive; 0.287
Luster: Adamantine to metallic.
Dispersion: Very high.
Dichroism: Strong but variable.
Occurrence: Found in igneous rocks, pegmatites, metamorphic rocks and limestones.
Notes
Common as inclusion in quartz (rutilated quartz) and a wide variety of gems; made synthetically as diamond simulant with yellow tint, high DR and strong dispersion; faceted for collectors.
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