Leuco-garnet - from the Greek leukos meaning white or colourless - is a trade and mineralogical term for colourless or near-colourless garnet, most commonly the grossular species (Ca3Al2Si3O12) but occasionally the spessartine and andradite species in chemically near-pure form. The colourless variety is uncommon in jewellery use but is significant gemmologically as a reference for the grossular end-member and as a collector species; it is sometimes also encountered as a refractive substitute or imitation for diamond, particularly in older Indian and Sri Lankan trade.

Composition and origin

Garnet is colourless when its chemistry approaches the pure end-member of one of the principal species without the chromophore impurities that produce the species' characteristic colours - manganese (spessartine pink-orange), iron (almandine red, andradite green-yellow), chromium and vanadium (tsavorite green grossular), titanium and iron (Ti-rich green andradite). When grossular grows in a host environment chemically depleted of these chromophores - typically a calcium-aluminium silicate skarn or rodingite - the result is a transparent, near-colourless to faintly tinted material.

Principal sources of leuco-garnet are Sri Lanka (the historical source of much trade material), Tanzania, Mexico (Lake Jaco area, where colourless and pale grossular has been recovered alongside the more famous green grossular and pink mexican garnet), and Pakistan. Sizes are usually small (under 5 carats), though larger stones of up to 20 carats have been documented in the Sri Lankan and Mexican trade.

Properties and identification

Leuco-garnet (grossular) shows:

• Refractive index 1.730-1.760 (single value, isotropic)
• Specific gravity approximately 3.55-3.65
• Mohs hardness 7-7.5
• Inert to weakly fluorescent under UV

The high refractive index gives the stone good brilliance when well cut, which has historically led to its use as a low-cost diamond imitation in older trade. Modern gemmological identification is straightforward - the isotropic character, refractive index and density are diagnostic - but in poor light or in older settings without paperwork, leuco-garnet can be confused with diamond, white sapphire, white zircon, white topaz or even high-quality glass. A loupe and refractometer at the bench will resolve any doubt.

Trade and use

Leuco-garnet is collected primarily as a mineralogical specimen rather than worn as jewellery, and the gem trade in colourless garnet is small. Cut stones appear in collector parcels and occasionally in vintage jewellery from Sri Lanka and India, where the material was used historically for affordable diamond-substitute mountings. Pricing is modest; well-cut clean stones in larger sizes command three-figure-per-carat prices for mineralogical interest, and the species offers no commercial substitute role in the modern market, where cubic zirconia, moissanite and laboratory-grown diamond cover the diamond-substitute role at lower cost and with better dispersion.