Grossular is the calcium aluminum member of the garnet group, and it comes in more colors than almost any other garnet: colorless, green, yellow, orange, brown and pink. Its two best-known gem varieties are tsavorite, a bright green garnet from East Africa, and hessonite, the orange-brown "cinnamon stone." Grossular is hard, has no cleavage and is rarely treated.
What grossular is
Grossular's ideal formula is Ca3Al2(SiO4)3, with calcium in the X site and aluminum in the Y site. It belongs to the ugrandite series of garnets with uvarovite and andradite, and it mixes with andradite in nature. The name comes from Ribes grossularia, the Latin botanical name for gooseberry, because the first described crystals were pale green like the fruit.
Pure grossular is colorless. Trace elements give it color: vanadium and chromium make it green, iron and manganese give yellow, orange and brown. It is cubic, singly refractive and has a refractive index around 1.73 to 1.76. Hardness runs 6.5 to 7.5, and specific gravity is about 3.6 to 3.7. The color in gemstones page explains how trace elements color allochromatic gems like grossular.
Tsavorite
Tsavorite is transparent green grossular colored by vanadium, sometimes with chromium. Colors range from light mint green to a deep, saturated green that rivals fine emerald. It is usually cleaner and brighter than emerald, with higher refractive index and more brilliance, and it does not need oiling.
Discovery in East Africa
Green grossular was found in northeastern Tanzania in 1967 by the British geologist and gem prospector Campbell Bridges. He later found more in southern Kenya near Tsavo National Park in the early 1970s and began mining there. Tiffany & Co. introduced the stone to the public in 1974, and the name tsavorite was chosen after the park. The deposits lie in an old metamorphic belt that runs along the Kenya-Tanzania border, where graphite-bearing gneisses host the garnet. The metamorphic rocks and gemstones page explains how such deposits form.
Tsavorite has also been found in Madagascar and, in smaller amounts, Pakistan.
Size and value
Tsavorite crystals tend to be small, and most faceted stones weigh under 2 carats. Clean stones over 5 carats with fine color are rare and command high prices per carat. The most valued color is a vivid, slightly bluish green with medium to medium-dark tone; very dark stones lose brightness. Tsavorite is not known to be treated. Mint grossular, a lighter green from Tanzania, is less expensive and has become popular for its brightness.
Hessonite
Hessonite is orange to reddish brown grossular colored by iron and manganese. It is sometimes called cinnamon stone for its warm brown-orange color. Under magnification, hessonite has a distinctive "treacle" or "heat wave" appearance, a swirled, oily look caused by many small included crystals of apatite or calcite and by growth features. That texture helps separate it from other orange gems.
Sri Lanka has been the classic source of hessonite for many centuries, from its gem gravels, and India has also produced it. Hessonite has been carved and set since antiquity. In Indian astrology, hessonite (gomed) is one of the nine stones of the navaratna arrangement, representing the shadow planet Rahu. That is a cultural tradition. The jewelry of India page explains the navaratna, and gems of Sri Lanka covers the source.
Hessonite is also found in Canada (Quebec), Brazil, Madagascar and Tanzania.
Mali garnet
Mali garnet is a mix of grossular and andradite discovered in western Mali in the mid 1990s. Its colors run from yellowish green to yellow and brown. Because of its andradite content, Mali garnet has higher refractive index and dispersion than pure grossular, giving it noticeable brilliance and some fire. It is usually small and is sold as an affordable bright garnet. The andradite side of this mix is covered on the andradite garnet page.
Other colors
Grossular also occurs in pale yellow and golden tones, colorless (sometimes called leuco-garnet), and pink to raspberry red from Mexico. Mexican pink grossular forms small, attractive crystals in marble and is more common as mineral specimens than gems.
Hydrogrossular
Hydrogrossular is a massive, translucent to opaque grossular that contains hydroxyl in place of some silica. It is often green, colored by chromium, and sometimes pink or white. Much hydrogrossular comes from South Africa, where it was traded under misleading names such as "Transvaal jade" or "South African jade." It is not jade. It is used for beads, cabochons and carvings. The jade page explains how true jadeite and nephrite differ.
How grossular forms
Most grossular forms in metamorphosed limestones and calcium-rich rocks, especially skarns, where hot fluids from an intrusion react with carbonate rock. Tsavorite forms in metamorphosed sedimentary rocks rich in graphite and vanadium. Hessonite and other colored grossulars wash out of these rocks into river gravels, which is why Sri Lanka's illam pits yield them.
Identification
Tsavorite can be confused with demantoid, emerald, chrome tourmaline and chrome diopside. Single refraction, a refractive index around 1.74, and the lack of horsetail inclusions or strong dispersion separate it. Hessonite's swirled internal texture and its refractive index set it apart from spessartite, zircon and citrine. The instruments for these tests are described on the gemological instruments page.
Wear and care
Grossular is durable for most jewelry. It is hard enough for rings and has no cleavage. Hessonite with many inclusions is slightly more fragile. Clean grossular garnets with warm soapy water and a soft brush. Ultrasonic cleaning is usually safe for clean, untreated stones, but avoid it for heavily included hessonite. The jewelry cleaning guide has more.
Frequently asked questions
What is the difference between tsavorite and emerald?
Tsavorite is a green grossular garnet colored by vanadium, while emerald is beryl colored by chromium or vanadium. Tsavorite is usually cleaner, brighter and untreated.
Why is it called tsavorite?
The name comes from Tsavo National Park in Kenya, near where the stone was mined in the early 1970s.
What is hessonite?
Hessonite is orange to brownish red grossular garnet. Its swirled, oily internal look is a classic identifying feature.
Is hydrogrossular a type of jade?
No. Hydrogrossular is a massive garnet sometimes sold as "Transvaal jade," but it is a different mineral from jadeite and nephrite.