Category Archives: Gemstone Overview


23ct Rhodolite, 20ct Tsavorite and 32ct Spessartite Garnets
Photo by: Mark Mauthner

The name “garnet” is derived from the Latin “granatus”, referring to the resemblance garnet crystals share with pomegranate seeds

Garnet has a history of appreciation dating back thousands of years, from the time of the Egyptian pharaohs to the Greco-Roman period and through the Middle Ages. It was one of the most favored gems of religious, political and social leaders to signify status

The garnet group of gems is made up of the end-members pyrope, almandite, spessartite, grossular and andradite

Deep red Bohemian pyrope garnets, discovered in the early 16th century, helped increase availability of garnet for the middle class, and thus made it a more mainstream gem until the late 1800’s

Fun Fact: the word “pyrope” stems from the Greek words meaning “fire eye”

40.66ct Rhodolite from Tanzania
Photo by: Robert Weldon

Rhodolite garnets are a pyrope-almandine mixture that range in color from purple to purplish-red. This 40.66ct purple rhodolite from Tanga, Tanzania, is an extremely large example of this material

Grossular Garnet Rough and Cut
Photo by: Jeff Scovil

Garnet has a high to very high refractive index range, from pyrope and grossular at approx ~1.74 to rhodolite at ~1.76 to spessartite at ~1.80 and andradite at ~1.88. Gemologists use refractive index, specific gravity, absorption spectra and inclusions to differentiate garnet’s end-members and varieties

8ct Tsavorite
Photo by: Robert Weldon

Tsavorite is the rich green variety of grossular garnet discovered by Campbell Bridges in 1967 in Tanzania, and again 3 years later in Kenya

Demantoid Garnets from Ural Mountains in Russia
Photo by: Robert Weldon

Andradite garnets range in color from brown to the vivid green of highly sought after Russian demantoids. The cause of this vivid green color is due to trace amounts of chromium, while iron gives it a more yellowish appearance. The discovery of Russian demantoid garnet in 1854 added an exciting new color variety to the garnet family, which designers eagerly embraced. The initial source was quickly depleted but left a lasting mark on the industry that allowed for its resurgence when material reentered the market after the Cold War. Today, commercial quantities of demantoid are available from Russia, Namibia and Madagascar

Fun Fact: Andradite’s dispersion is 0.057, which is higher than that of a diamond at 0.044

Demantoid Crystal in Matrix
Photo by: Jeff Scovil

Garnets typically form under metamorphic or metasomatic conditions in which preexisting igneous or sedimentary rocks are altered by heat and pressure due to tectonic forces or contact with an igneous intrusion

Red hessonites (above right), a variety of grossular, enjoyed increased production in recent years from Southern Sri Lanka, providing a more affordable alternative to red spessartite

Spessartite Garnet Crystals and Gems from Tanzania & Nigeria
Photo by: Jeff Scovil

Gem quality spessartite crystals are found within metamorphosed schists. These crystals occur as euhedral or subhedral rhombic dodecahedrons (24 faces) which can sometimes look almost spherical

Namibian “Mandarin” Spessartites ranging from 0.92ct to 10.25ct

These vivid orange spessartites are extremely rare, usually occurring in sizes less than one carat. They were discovered by a farmer in 1992 along the Namibian border with Angola. The cause of this remarkable color is Manganese

12.18ct Malaia Garnet

Malaia garnets are a pyrope-spessartine mixture that often exhibit a color shift or even a color change, like the gem above. Recent increase in supply from Tanzania and Madagascar has helped make this a popular garnet in contemporary jewelry design

33.63ct Rainbow Garnet

Rainbow garnet from Alamos Mexico, New Mexico and Japan exhibits a play of color due to an outer layer of growth that creates diffraction grating-like interference colors

A new variety of garnet was discovered in Mali, West Africa, in 1994. Grossular-andradite, known in the trade as “Grandite Garnet” or “Mali Garnet”, has a chemical composition of (Ca,Fe)3Al2(Si04)3 and a refractive index range of 1.752 – 1.782. The color of most of the material ranges from yellowish-green to rich green to brown

Click below to see the Gems & Gemology, Fall 1995, article co-authored by Edward Boehm

Gem-Quality Grossular Andradite: A New Garnet From Mali


Violetish-Blue and Green Tanzanite (11 – 23ct)
Photo by Robert Weldon

Celebrating December’s Birthstone

Maasai traditions associate the color blue with new life, so it is only fitting that tanzanite, a stone discovered by Maasai herders in 1967, is the traditional gift for new mothers. It is, truly, the ultimate Birthstone.

A relative newcomer to the gem world, tanzanite has made a lasting mark on the jewelry industry. Mined only in one small area of a few kilometers in the Merelani hills, approximately 70 km Southeast of Arusha and Northwest of Mount Kilimanjaro, this rare stone is coveted by collectors and jewelers alike.

The rarest color of tanzanite is green, colored by trace amounts of chromium. Discovered in 1991, the local miners called it “combat”
because of its yellowish-green to bluish-green color. It is still considered one of the most collectable gems from Tanzania.

Read more about green tanzanite in this article by Edward Boehm and Dr. Barot – Gems and Gemology, Spring 1992

Maasai Warriors

Tanzanite was discovered in 1967 when wildfires engulfed the Merelani hills at the foot of Mount Kilimanjaro, naturally heat treating the material and exposing their striking blue-violet color.  A group of Maasai herders saw the exquisite stones and reported their findings to Portuguese geologist, Manuel D’Souza, who introduced this new discovery to the western world.

Tanzanite and Tsavorite Crystals
6ct Blue Tanzanite

Tanzanite’s strong pleochroism causes it to display a multitude of colors, primarily blue and violet. Depending on the lighting, a single gem can present as blue, violet and burgundy. This is due to the trace amounts of vanadium and/or chromium present during its formation. As with many gems, larger stones tend to appear in a deeper color than their small counterparts.

Independent artisanal miners in Block D 

Most tanzanite has to be heated to improve its color, which reduces or removes its reddish brown properties or improves color saturation.  Recently, natural color material has gained favor because of the strong pleochroic colors that can be manifested through skillful cutting.

8ct Pink Tanzanite