Gem Focus – December 2021

Posted on December 8, 2021 by Çiğdem Lüle, PhD, FGA, GIA GG, DGA

Hydrogrossular Garnet; a lesser-known garnet species better known by its trade names.

The garnet group minerals include some marvelous species that always appeal to different facets of the gem market. Large, transparent, and red species; such as almandine, pyrope, and spessartine have been fashioned as beads and cabochons for millennia. Red garnets were much more popular, perhaps due their availability and transparency for a very long time. Strangely enough, the oldest recorded engraved garnet seal is not a red one, it is actually another less assuming species, hydrogrossular which is translucent and comes in shades of green, yellow, brown, and pink. The light yellowish green hydrogrossular cylindrical seal is from Mesopotamia, Neo-Assyrian to be precise, and dated to the 8th Century BC.

Pink graduating to green hydrogrossular garnet from South Africa.
Courtesy of Bonham’s.

Unlike other gem garnets, hydrogrossular garnet comes in massive form and different color variations. Most common ones are yellowish green to green. In fact, it has a misnomer as “Transvaal Jade” when found in South Africa. Interestingly, the same locality can produce pink hydrogrossular and it is not unusual to see color zoning in this material. Large pieces with reasonable durability make this garnet variety a great candidate for carvings. Some deep green varieties may have black inclusions similar to nephrite jade with evenly colored translucent structure and yield into small engravings in jewelry.

Masquerading as jade is not unique to South African material. Many localities such as China, Pakistan, and California, US, can produce green hydrogrossular garnet. High luster and translucent structure might be a good disguise but a simple refractive index check and specific gravity testing, when possible, help the gemologists to separate these materials apart.

A rock crystal, diamond and hydrogrossular garnet flower brooch.
Courtesy of Bonham’s.

Collectors and gemologists should also be aware of the Californian hydrogrossular mixed with vesuvianite which is a different mineral with very similar properties to garnet. Many gemologists are familiar with vesuvianite as idocrase so it is important to know that these two mix well in their massive form from California.

Garnet group gemstones have been increasingly popular in more recent years. The diversity of colors has generated a lot of interest from buyers priced out of the market for other more costly gemstones.  Numerous species, colors, and sizes in affordable prices appeal to many participants of the gem market. While transparent varieties are better known, it is pertinent to be able to identify lesser-known garnets in jewelry. After all, unusual always sells.

Image in Header: Hydrogrossular garnet seal, 4.3cm tall, an adoration scene of Goddess Ishtar. Neo-Assyrian epoch, 8th Century BC. Courtesy of The Trustees of the British Museum.

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