Namibian demantoid is demantoid garnet from Namibia, principally from the Erongo region, discovered in the mid-1990s. Demantoid is the green chromium-bearing variety of andradite garnet (Ca₃Fe₂Si₃O₁₂) and is one of the rarest and most valuable garnet varieties. The Namibian discovery transformed the demantoid market by providing a substantial alternative source to the historic Russian Ural production, with material that rivals or matches the original deposit on most quality factors.

Geological setting

The Namibian deposit lies in altered ultramafic rocks of the Erongo region, distinct geologically from the serpentinite-hosted Russian Ural deposit but producing demantoid with broadly similar chromophore chemistry. Chromium is the primary colour-causing element in fine Namibian and Russian demantoid, producing the saturated bottle-green to slightly yellowish-green that defines the variety. Iron content modulates the colour and contributes to the slight yellow modifier seen in some Namibian stones.

Optical properties

Demantoid has a refractive index of about 1.880 to 1.889 — the highest of any garnet variety — and a dispersion (fire) of 0.057, exceeding diamond's 0.044. The combination produces the strong adamantine lustre and pronounced fire that distinguish demantoid in the cut. Hardness sits at 6.5 to 7.0 on the Mohs scale, which limits demantoid to careful settings rather than aggressive everyday wear.

Inclusions

Russian Ural demantoid is famous for its 'horsetail' inclusion — a radiating fibrous bundle of byssolite (a fibrous amphibole) that splays from a central point inside the stone. The horsetail is one of the few inclusions that adds rather than detracts from value, and Russian connoisseurs prize stones with well-formed horsetails. Namibian demantoid also produces horsetail-like inclusions on occasion, though they are typically less prominent and less consistently present than in the Russian material. Namibian stones may show fingerprint-like fluid inclusions and other features rather than dominant horsetails.

Size and supply

Namibian demantoid typically appears in faceted sizes under 2 carats, with stones above 5 carats rare and commercially significant. Production has been intermittent, with periods of strong supply followed by quiet stretches as accessible material is worked through. The deposit has provided enough material to make demantoid more available to the modern jewellery trade than at any previous time, though it remains a rare and sought-after variety.

Trade and value

Namibian demantoid trades at meaningful prices but generally below comparable Russian Ural material on the high end of the market — Russian provenance carries an additional premium, particularly when accompanied by laboratory origin opinion. Namibian stones are nonetheless premium gems: top-quality faceted demantoid above 1 carat regularly trades in the high three- to mid-four-figure US dollar per carat range, with exceptional stones higher still. See also demantoid, Erongo, horsetail inclusion.

Further reading

• GIA — garnet reference
• GIA Gems & Gemology — demantoid articles
• ICA — demantoid trade reference