Scheelite

Chemistry: Calcium tungstate.
Crystal system: Tetragonal; dipyramids (may look like octahedral) or tabular crystals, also granular masses.
Color: Transparent to translucent; colorless, brown, orange, yellow, purple.
Hardness: 4.5 - 5
Cleavage: Perfect: 1 direction: fracture: splintery, sub-conchoidal to uneven.
Specific gravity: 5.9 – 6.1
Refractive index: 1.918 – 1.934; Uniaxial positive; 0.016
Luster: Vitreous to adamantine.
Dispersion: Medium
Dichroism: Strong.
Occurrence: Metamorphic, hydrothermal and pegmatites; Mexico, USA, Italy, UK.

Notes
May be dyed to look like many other stones; fine yellow may look like fancy diamond, but DR; also made synthetically; fluorescence: strong blue in short wave, but no reaction in long wave; may show didymium spectral lines in yellow and green; faceted for collectors.