Madagascar demantoid is the chromium- and iron-coloured andradite garnet recovered principally from the Antetezambato deposit on the north-west coast and from related occurrences around Ambanja. The discovery of the Antetezambato find in 2009 added a major new source to a market that had been dominated for over a century by Russian Ural demantoid and, more recently, by Namibian and Iranian material.

Material character

Antetezambato stones range from a yellowish green through fine vivid green to bluish green, with the most prized colour resembling the saturated grass green of the best Ural goods. Calcium-rich skarn is the host environment, and the deposit produced an unusual proportion of clean stones in commercial sizes; cut goods above five carats appeared regularly during the production peak, a yield difficult to match from Russian sources.

Refractive index sits at the upper end of the garnet range, often above 1.88, and the dispersion of 0.057 is famously higher than diamond, accounting for the strong fire visible in well-cut stones. Inclusions include the classic horsetail bundles of byssolite seen in some material, healed fingerprints and characteristic skarn-mineral inclusions; horsetails are less common in Madagascan stones than in Russian.

Trade and identification

Production from Antetezambato declined sharply after the initial rush, but parcels continue to surface, and laboratory origin determination is achievable through trace-element chemistry, particularly chromium-to-iron ratios. GIA and SSEF have published peer-reviewed work documenting the Antetezambato chemistry that allows separation from Russian, Namibian and Iranian goods. Heat treatment is uncommon for fine demantoid; the colour is generally accepted as natural.

For dealers, Madagascan demantoid is regarded as a credible alternative to Ural material at a more accessible price point, while top stones from the early production rival Russian quality in saturation and clarity.