Pyrope

Pyrope
A gem-quality, emerald cut pyrope.
General
Category	Nesosilicate
Formula (repeating unit)	Mg3Al2(SiO4)3
Strunz classification	9.AD.25
Crystal system	Cubic
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Ia3d
Identification
Color	Red. Some varieties are very dark, almost black, while others can take tones of purple. Some chromium-rich pyropes are thermochromic, becoming green when heated
Crystal habit	Euhedra typically display rhombic dodecahedral form, but trapezohedra are not uncommon, and hexoctahedra are seen in some rare samples. Massive and granular forms also occur.
Cleavage	None
Fracture	Conchoidal
Mohs scale hardness	7 - 7.5
Luster	greasy to vitreous
Streak	White
Specific gravity	3.78 (+.09 -.16)
Polish luster	vitreous
Optical properties	Single refractive, often anomalous double refractive
Refractive index	1.74 normal, but ranges from 1.714 to over 1.742
Birefringence	Isotropic, appears black in cross-polarized light
Pleochroism	none
Ultraviolet fluorescence	inert
Absorption spectra	broad band at 564nm with cutoff at 440 to 445nm. Fine gem quality pyropes may show chromium lines in the red end of the spectrum
Solubility	Insoluble in water, weakly soluble in HF
Mineral association	Olivine, pyroxene, hornblende, biotite, diamond
References

The mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for fire and eye. Despite being less common than most garnets, it is a widely used gemstone with numerous alternative names, some of which are misnomers. Chrome pyrope, and Bohemian garnet are two alternative names, the usage of the latter being discouraged by the Gemological Institute of America. Misnomers include Colorado ruby, Arizona ruby, California ruby, Rocky Mountain ruby, Elie Ruby, Bohemian carbuncle, and Cape ruby.

The composition of pure pyrope is Mg₃Al₂(SiO₄)₃, although typically other elements are present in at least minor proportions—these other elements include Ca, Cr, Fe and Mn. Pyrope forms a solid solution series with almandine and spessartine, which are collectively known as the pyralspite garnets (pyrope, almandine, spessartine). Iron and manganese substitute for the magnesium in the pyrope structure. The resultant, mixed composition garnets are defined according to their pyrope-almandine ratio. The semi-precious stone rhodolite is a garnet of ~70% pyrope composition.

The origin of most pyrope is in ultramafic rocks, typically peridotite from the Earth's mantle: these mantle-derived peridotites can be attributed both to igneous and metamorphic processes. Pyrope also occurs in ultrahigh-pressure (UHP) metamorphic rocks, as in the Dora-Maira massif in the western Alps. In that massif, nearly pure pyrope occurs in crystals to almost 12 cm in diameter; some of that pyrope has inclusions of coesite, and some has inclusions of enstatite and sapphirine.