Prasiolite — The Green Quartz That Was Almost Always Heat-Treated Amethyst
Prasiolite — The Green Quartz That Was Almost Always Heat-Treated Amethyst
A transparent green quartz produced by heating amethyst from specific deposits, marketed for decades under the misleading trade name green amethyst
Prasiolite is the gemmologically correct name for transparent green quartz, ranging from a delicate pale yellowish-green to a saturated medium green. The overwhelming majority of commercial prasiolite is produced by heating amethyst from a small number of deposits — most importantly the Montezuma mine in Minas Gerais, Brazil — at controlled temperatures. Naturally green quartz of this type does occur but is exceptionally rare; the trade has been built almost entirely on the heat-treated material, with disclosure of treatment a recurring point of friction in jewellery retail. The misleading trade name green amethyst, although still widely seen on online marketplaces, is rejected by GIA, AGTA, CIBJO, and the FTC's gem nomenclature guidance.
Composition and origin of colour
Prasiolite is silicon dioxide, SiO2, the same chemistry as quartz, amethyst, citrine, smoky quartz, and rose quartz. Hardness is 7 on the Mohs scale, specific gravity 2.65, and refractive indices 1.544 to 1.553 with a uniaxial positive optic character. The green colour is attributed to a charge-transfer mechanism involving iron in particular oxidation states (Fe2+ in specific structural sites), with the precise colour mechanism still the subject of refinement in the gemmological literature; it is not the iron-cluster mechanism that produces amethyst's purple, which is destroyed by the heat treatment that converts amethyst to prasiolite.
Heat treatment is performed at controlled temperatures between roughly 470 and 600 degrees Celsius, with the precise schedule depending on the parent material. Not every amethyst will turn green on heating — most amethyst yields citrine — and the deposits that produce stable prasiolite on heating are limited. Montezuma is the principal source; secondary production has come from Arizona (Four Peaks area), Poland, and Russia. The colour is stable under normal wear conditions but can fade with prolonged exposure to strong light or sustained high temperature.
Naturally green quartz
Untreated naturally green quartz exists but is exceptionally rare in gem-quality form. Documented natural prasiolite has come from a handful of localities, with stones identified in the Brazilian Bahia state, Lower Silesia in Poland, and the Thunder Bay area in Ontario. The colour mechanism in natural material is variable; some natural green quartz owes its colour to fine inclusions of green minerals (chlorite, actinolite) rather than to the lattice mechanism responsible for treated prasiolite, and is sometimes classified separately as included quartz rather than prasiolite proper.
Identification and disclosure
The treated and untreated forms are visually similar, and laboratory identification is required for any high-confidence statement of origin. Standard gemmological tests confirm species (RI, SG, optic figure) but do not differentiate treated from untreated material; the distinction requires advanced spectroscopy at a competent laboratory, and untreated stones are typically accompanied by a laboratory report when they reach the trade. Treatment must be disclosed under FTC and CIBJO guidance; the LMHC (Laboratory Manual Harmonisation Committee) and GIA both publish the standard treatment-disclosure code for prasiolite (heat treatment).
The trade name green amethyst is misleading because amethyst is by definition purple; once heated to green, the stone is no longer amethyst. Vermarine and green quartz have been used as alternatives but are non-standard. AGTA's gem nomenclature code requires prasiolite for the species and disclosure of any treatment.
In the trade
Prasiolite is a commercial volume gem, available in clean stones up to 50 carats and beyond, in standard faceted shapes (oval, round, cushion, pear, emerald cut). The colour is gentle and often pales toward yellowish-green at smaller sizes, which means buyers seeking saturated green do better with stones in the 5-carat-plus range. Pricing is modest — comfortably below amethyst per carat for clean material — which makes prasiolite a useful designer stone for accessible green jewellery and an alternative to peridot for those seeking a cooler, less olive green tone.
Untreated natural prasiolite, when it appears with a competent laboratory report, commands significant premiums and trades primarily through specialist channels. Buyers should ask explicitly whether the stone is heat-treated, and treat unanswered or evasive responses as confirmation of treatment.
Care
Prasiolite is durable for general jewellery use at hardness 7. Mild soap and warm water are the standard cleaning method; ultrasonic cleaning is generally safe for stones without visible inclusions, although prolonged exposure to high temperature should be avoided to preserve colour stability. Prolonged direct sunlight exposure can fade the colour over years.