Idocrase

Idocrase was first discovered on the Italian volcano, Vesuvius, as small, perfect crystals. Also called vesuvianite, it may be red, yellow, green, brown, or purple. It is seldom used in jewellery, but it may be cut for collectors. Crystals are usually thick prisms with a square cross-section.

There are several varieties: californite from California (USA) is green; rare, blue cyprine is found in Norway; yellowish green xanthite is from New York (USA); green wiluite crystals are from the former USSR. Other localities include Austria, Canada, Italy, and Switzerland.

Vesuvianite may be confused with demantoid garnet, diopside, epidote, smoky quartz, tourmaline, zircon, and peridot.

Idocrase Properties

Chemical Composition: 
Complex silicate of calcium and aluminium. Ca10Mg2Al4(SiO4)5(SiO7)2(OH)4 and Be, Cu, Cr, Mn, Na, K, Ti, B, U, Th, Zn, Sn, Sb and rare earth elements.
Colors / Varieties: 
  • Colorless, green, brown, white, yellow, brownish, red, blue, bluish green, pink, violet, sometimes color zoned.
  • Californite: massive green
Crystal System / Forms: 
Tetragonal System / Well formed crystals - prismatic with pyramidal terminations, granular, massive, and intergrown with grossular garnet.
Hardness: 
6 - 7
Specific Gravity: 
3.32 - 3.47
Cleavage / Fracture: 
Indistinct / Conchoidal
Optic Character: 
Anisotropic, D.R.; Uniaxial positive or negative
Lustre: 
Vitreous to resinous
Refractive Index / Birefringence: 
1.713 - 1.718 / 0.005; Range: 1.700 - 1.720
Pleochroism: 
Weak
Dispersion: 
0.019
Magnification: 
Crystalline and fingerprint inclusions.
U.V. Fluorescence: 
Inert.
Spectrum: 
Strong line at 461nm, weaker at 528nm
Cause of Color: 
  • Iron: green, yellow, brown
  • Copper: blue
Specific Tests: 
Attacked by hydrochloric acid.
Simulants (with separation tests): 
Hydrogrossular garnet (spectrum, R.I.), Jade (R.I., spectrum, structure), Epidote (birefringence, pleochroism), Hypersthene (birefringence, spectrum), Kornerupine (R.I., spectrum, pleochroism), Enstatite (R.I., spectrum), Peridot (R.I., birefringence, spectrum), etc.
Geological Occurrence: 
Contact metamorphic deposits, especially in limestone and dolomites, alkalic rocks; regionally metamorphosed rocks.
Sources: 
Canada, Pakistan, Italy, Siberia, Switzerland, U.S.A. (California), India
Cuts & Uses: 
Facetted cuts, cabochon, beads, carving, etc.