Andradite Garnet: Gemstone Information 

The Name Andradite Garnet approved by the CNMMN of the IMA. Synonyms are allochroite, aplome, calcium-ferrigranat, polyadelphine and polyadelphite. Andradite Garnet is named after the Brazilian statesman and geologist Jose Bonifacio d’ Andrada e Silva (1763–1838), who first described the mineral.
Frequently found in metamorphosed limestones and metamorphosed impure carbonate-bearing rocks; particularly in metosomatic skarns. The light-coloured andradite varieties topazolite and demantoid occur mainly in chlorite schists and serpentinites respectively. The darker andradite variety melanite and the related schorlomite occur mainly in alkaline igneous rocks such as nepheline syenites and phonolites. Andradite also occurs in metamorphosed igneous rocks, and additionally as detrital grains in placer deposits. Notable localities for gem-quality andradite include: Russia (Ural Mts), USA(Arizona), Namibia (Erongo Mts) for green (vanadium rich–chromium poor) andradite, some of which may be considered demantoid; Iran (Kerman), Afghanistan (Khost, Kunnar) and Pakistan (Kashmir) for demantoid; USA (California), Italy (Piedmont) and Switzerland (Zermatt) for topazolite; Greenland, Norway, Sweden, Germany, Uganda, Madagascar, Sri Lanka, Russia and Zaire for andradite of non-specific colour variety or melanite. Semi-transparent light apple-green demantoid with significant grossular/ hydrogrossular content has been reported from Kamchatka and Chukotka, Soviet Union.
Andradite generally contains significant amounts of the grossular molecule, and a continuous isomorphous series exists between these two components. This has been termed the grandite series. A second series exists with schorlomite that includes melanite (titanium andradite). It is sometimes in solid solution with spessartine and uvarovite. Consequently the physical and chemical properties and appearance can vary appreciably.

Refractive Index
The calculated value of the end-member is 1.889. Actual values will vary due to isomorphous replacement and literature values for 50–100% andradite range from 1.82 to 1.89 respectively. Melanite, with its high titanium content, gives elevated RI values up to 1.97 where Fe3+ > Ti. Demantoid values are reported as ranging from 1.880 to 1.889; the higher value relating to dark-green material. Topazolite RIs ranging from 1.84 to 1.89 have been recorded for Alpine material.
Pure end-member (calculated) 3.859. Extracting values from the literature, where chemical information is provided, indicates that for ugrandite garnets containing 50% andradite molecule the lower limit of density approximates to 3.77. Measured values for SG normally range from 3.82 to 3.85. It should be borne in mind that inclusions could affect measured density and, if in abundance, give rise to skewed values.
Hardness on Mohs’ scale ranges from 6.5 to 7.5 with demantoid at the lower end of this range.

Very high at 0.057; greater than that of diamond (0.044) or titanite (0.051). The effect is partially masked by body colour especially in intensely coloured demantoid.
Occurs in various hues: red, brown, yellow (topazolite) and green (demantoid) and shades in between these; also as black (melanite). Also reported as colourless and grey. Tsavorite-like coloured andradite has been reported from Madagascar. Colour Varieties: Demantoid colour arises from the presence of small amounts of chromium. Whilst the colour of demantoid never approaches that of the finest emerald, favourable comparison can be made especially in vividly coloured stones that are not overly dark or contain yellow hues.
Chemical analysis of demantoid from Kashmir, Pakistan, showed relatively pure andradites with 0.25 wt% of Cr2O3. The demantoid from Panshir Valley, Kunnar, Afghanistan, is transparent but very dark green. The green andradite from Farm Tubussis, Erongo Mountains, Namibia, has only minor chromium but a significant vanadium content and is being traded as demantoid.
Topazolite was named after its similarity to yellow topaz but is now used to include also more orangy material. Some andradite from the Urals is a strong yellow component and is closer to topazolite than demantoid.
Melanite is from the Greek word melanos meaning black, referring to its colour. This opaque material was historically used for mourning jewellery, or as inlay.
Significant, and diagnostic when seen, are wispy, horse-tail like, fibrous inclusions of chrysotile (serpentine asbestos) inclusions that historically had been referred to as byssolite, a now obsolete term, or fibrous asbestiform actinolite. Seen in demantoid from Russia, Eritrea and Italy but not Namibia. Opaque chromite crystals may form the nucleation point for the chrysotile. Tiny fluid inclusions are seen in Namibian demantoid, also negative crystals with two phase inclusions and pronounced angular or straight transparent growth zoning which rarely may be iridescent (‘rainbow graining’). Acicular diopside crystals have been reported as inclusions in demantoid from the Urals.
Yellowish green to emerald-green demantoid from Kashmir, Pakistan, had acicular, fibrous and felt-like mineral inclusions that were identified as chrysotile. Partially healed fractures and liquid inclusions plus small unidentified needle-like inclusions and transparent crystals have been reported from andradite from the Yemen. Healed fractures, growth banding (best seen in polarized light) and two phase inclusions have been reported in Arizona andradite. Yellowish brown Namibian andradite had yellow needles.
Optical Effects: Chatoyancy, Asterism, Colour Change, ADR
Iridescent andradite from Sonora, Mexico, is sometimes referred to as ‘rainbow garnet’. The iridescence is caused by cylindrically stratified growth lamellae in the {211} and {110} sectors. The stratification is between An79% and An94%. Green andradite from Farm Tubussis, Erongo Mountains, can often have an unappealing colour shift towards brown in tungsten light. Some demantoid show a colour change from yellowish green to a bright
sparkling pastell green (daylight). Rarely, some show a colour change from green to orange.
Colour enhancement of demantoid has been achieved by heat treatment under reducing conditions. Brown and yellow hues are removed and green enhanced by converting some Fe3+ to Fe2+ Alferova M.S. pers comm.).