Amethyst garnet is a trade name applied to a particular range of purplish-pink to violet garnets, principally pyrope-almandine intermediates that approach in colour the violet of fine quartz amethyst. The term is informal and not standardised in laboratory reporting, but it appears in dealer parlance and in occasional auction descriptions for stones whose hue lies on the violet side of the rhodolite range. The name is potentially confusing because amethyst proper is a variety of quartz, an entirely different species from garnet, and conscientious sellers therefore use amethyst garnet only with explanatory context.

Composition and species

The garnet group is a family of isostructural silicates, with end-member compositions including pyrope (Mg₃Al₂(SiO₄)₃), almandine (Fe₃Al₂(SiO₄)₃), spessartine, grossular, andradite, and uvarovite. Most gem garnets are intermediate compositions between adjacent end-members. Rhodolite, named in 1898 by Hidden and Pratt for material from Macon County, North Carolina, is a pyrope-almandine intermediate with a characteristic raspberry to purplish pink. As pyrope-almandine compositions trend toward higher iron content and incorporate trace amounts of other transition elements, the colour can shift toward the violet end of the range, producing the stones marketed as amethyst garnet.

Sources

Amethyst-garnet material has been recovered from several deposits known for fine pyrope-almandine. The most significant sources include Mozambique, Madagascar, Tanzania, Sri Lanka, and the Umba Valley region of Tanzania-Kenya. Mozambique material from the Niassa region, brought to commercial prominence in the late 1990s, has produced some of the finest violet-tone garnets seen in the trade. Material from Idaho's Star Garnet District is occasionally pinkish purple but more commonly almandine-pyrope with characteristic asterism.

Properties

As a member of the garnet group, amethyst garnet has hardness 7 to 7.5, refractive index in the 1.74 to 1.79 range depending on composition, and specific gravity around 3.7 to 4.2. It is singly refractive, lacks cleavage, and accepts a fine polish. The combination of hardness, durability, and brilliance makes garnets in general well suited to ring wear, and amethyst garnet shares these advantages. The stone is unaffected by ordinary chemical exposure and tolerates ultrasonic and steam cleaning when free of fractures.

Treatment

Garnets are notable for being among the few major gem species that are not routinely treated. The colour and clarity of amethyst garnet are typically natural, requiring no heating, irradiation, or impregnation to reach the form in which it is sold. This is one of the species' significant trade attractions. Laboratory reports for fine garnets typically state no indications of treatment with confidence.

Distinguishing from amethyst proper

Amethyst, the variety of quartz, is fundamentally different. Quartz amethyst has hardness 7, refractive index 1.544 to 1.553, specific gravity around 2.65, and is doubly refractive. The two stones look superficially similar in violet hues but differ obviously under refractometer or in specific gravity testing. Amethyst proper is the modern February birthstone; amethyst garnet, despite the trade name, is not.

In the trade

Amethyst garnet appeals to clients who like the purple-violet hue of fine amethyst but want the brilliance and refractive return of garnet. Compared with rhodolite, amethyst garnet trades at a modest premium when the violet tone is clean and saturated and the stone is well cut. We use the term sparingly in our own merchandising, generally preferring violet rhodolite or purple pyrope-almandine to avoid any species confusion.

Further reading

• GIA — Garnet reference
• GIA Gems & Gemology — garnet articles