Hydrogrossular is a hydrous, opaque to translucent calcium-aluminium garnet of the grossular series in which some of the silicate tetrahedra are replaced by hydroxyl groups. Its general formula is conventionally written Ca3Al2(SiO4)3-x(OH)4x, with x typically between 0.2 and 1. Where transparency is preserved the species is simply grossular; with progressive hydration and accompanying microcrystallinity the material becomes translucent to opaque and acquires the appearance and feel of a fine jade.

Mineralogy

Hydrogrossular crystallises in the cubic system as a member of the garnet family, with the same isometric structure as pyrope, almandine, spessartine, grossular and andradite. In hydrous form the structure is microcrystalline and aggregate, granular under the microscope, with individual grains rarely larger than a fraction of a millimetre. The refractive index varies with composition and water content, generally falling in the range 1.70 to 1.73, which is a useful diagnostic against jadeite (1.66) and nephrite (1.60). Specific gravity sits between 3.36 and 3.55, again distinguishing the material from the principal jades.

Colours include green of varying saturation, deriving from chromium and minor iron substitutions and reminiscent of fine jadeite at higher saturation; pink to mauve, deriving from manganese; greenish-yellow; and white. Many fine specimens show veining or mottling between green and white, which the trade has occasionally exploited to imitate Imperial jade.

Trade names and history

The principal historical source is the Bushveld Igneous Complex in South Africa, where hydrogrossular forms in metasomatised contact zones between the layered intrusion and surrounding country rocks. South African material was widely sold in the early twentieth century under the trade name "Transvaal jade", a name that the GIA and CIBJO now consider misleading and that should be qualified or replaced by "hydrogrossular garnet" on every laboratory document or invoice. The term "Garnet jade" has also been used. Mexican, Burmese and New Zealand occurrences have produced smaller volumes of similar material.

Hydrogrossular has been carved into beads, cabochons, bowls, figures and architectural inlays. It cuts and polishes well, with a hardness around 6.5 to 7 on the Mohs scale and a tough granular structure that resists chipping. Carvers in southern Africa and East Asia have used it as a substitute when jadeite was scarce or expensive.

Identification

Identification is straightforward with standard gemmological equipment. Refractive index by spot reading on a refractometer, specific gravity by hydrostatic weighing, and observation of granular structure under the microscope distinguish hydrogrossular from jadeite, nephrite, serpentine, prehnite and the other green ornamental stones it has been sold as. FTIR spectroscopy confirms the hydroxyl absorption that defines the species, and EDXRF gives the calcium-aluminium chemistry. Magnification frequently shows characteristic black inclusions of magnetite or chromite, especially in the South African material, which form a distinctive granular pattern against the green ground.

Trade and care

Hydrogrossular is durable, takes a good polish and is well suited to carving and beadwork. It should not be sold or invoiced as "jade" without qualification. The correct trade language is "hydrogrossular garnet" with optional reference to historic trade names in parentheses. Care is straightforward: warm soapy water and a soft brush are adequate, and the stone is not acid-sensitive in normal jewellery use. Steam and ultrasonic cleaning are generally safe but caution is appropriate for stones with visible fractures or composite assembly.

For collectors and serious carvings buyers, well-cut hydrogrossular cabochons of saturated chrome-green colour and clean groundmass remain interesting in their own right, particularly given the durability of the material and the relatively modest prices at which it trades compared to fine jadeite. The species is also a useful reference for any working dealer who handles pre-1960 estate jewellery, where pieces sold as "Transvaal jade" or simply "African jade" will routinely turn out to be hydrogrossular when tested.