Hydrogarnet: Gemstone Information
Garnet-vesuvianite ranges from opaque to translucent, depending on quality and thickness when cut. Mostly cut cabochon-style, the gem could be confused with jade because of its mottled appearance and colours. The green portion of the gems have been identified as vesuvianite and the pink to red portions as hydrogrossular garnet. The
gems contain areas where the two species are mixed to such an extent they are almost impossible to separate gemmologically. The red and pink colours are attributed to manganese and the green to iron, not chromium as was previously suggested. California jade or californite, or vesuvian jade, is massive green grossular mixed with vesuvianite: RI 1.71–1.72, SG 3.25–3.32. Pseudomorphous substitution of wadalite (protoachtarandite) by hydrogarnet gives rise to white, predominantly hibschite pseudomorphs. Localities include Pakistan and California, USA.
Occurs principally in rodingites, contact-metamorphosed marls and skarn deposits. Also as layers coating andradite and grossular. ‘Transvaal jade’ is reported to be formed by combined contact-metamorphism and
metasomatism. Major localities include Rustenburg, South Africa;
Hydrogrossular: 3.3–3.48 and up to 3.523
Hibschite and katoite can be colourless, white, pale grey, light yellow, brown and pink. Transvaal jade can be green (chromium bearing) and pink (manganese bearing) to light red pink: also red brown, yellow blue to purple and sometimes brown.
Massive translucent to opaque grossular often with a hydrous component is known as ‘Transvaal jade’. The original material was reported to be found at Buffelsfontein and the adjacent farm of Turffontein, some 65 km west of Pretoria, South Africa. It is a compact homogeneous rock included with black specks of chromite and magnetite. The grey material is reported to contain up to 25% zoisite. RI range from 1.70 to 1.73; the lower value correlating with pink colour and increased hydrous values. Similarly SG correlates with colour and hydrous
values; pink material with elevated hydrous values varying from 3.36 to 3.41 and green from 3.42 to 3.55. Hardness varies between 7 (pink) and 8 (green). A weak chromium absorption spectrum, with strongest lines at 630 and 461 nm, has been reported from green specimens. Massive hydrogrossular fluoresces a strong orange-yellow under X-rays: a feature not seen in jadeite, nephrite and jade simulants. Bi-coloured green and pink material has been marketed as watermelon garnet.