Demantoid Garnet Gemstones – Properties, Origins, Treatments and Buying Guide

Demantoid Garnet is one of the most coveted and rare garnet varieties, prized for its vivid green colour, exceptional brilliance, and strong dispersion. Unlike ordinary green gemstones, demantoid garnet combines fiery sparkle with a natural, rich green hue.

In this guide we examine demantoid garnet from a professional gemmological and commercial perspective, covering its physical properties, crystal structure, treatments, origins, and how to identify fine-quality stones.

Demantoid Garnet is the green variety of the mineral andradite, a calcium iron silicate garnet (Ca₃Fe₂Si₃O₁₂). It is renowned for its vivid green colour and extraordinary fire, rivaling even diamonds in brilliance.

Crystals form in metamorphic rocks, typically in chromite-rich deposits, and are often small but of extremely high clarity, allowing exquisite gem cutting.

Demantoid Garnet has the chemical formula:

Ca₃Fe₂(SiO₄)₃

Primary composition:

Calcium
Iron
Silicon
Oxygen

Trace chromium is responsible for its vivid green colour. The proportion of chromium and iron controls the hue and intensity:

Low chromium = yellow-green
Moderate chromium = bright grass green
High chromium = intense emerald green

Demantoid Garnet belongs to the garnet mineral family, a group of nesosilicates.

Its crystal system is isometric (cubic), producing dodecahedral and trapezohedral crystals that contribute to the gem’s brilliance and fire.

Demantoid Garnet is prized for high refractive index, strong dispersion, and excellent clarity, giving it exceptional brilliance and “fire” unmatched in most other green gemstones.

Lustre
Vitreous, producing intense sparkle.

Transparency
Transparent; high clarity with few inclusions.

Refractive index
1.87 – 1.89, providing remarkable brilliance.

Specific gravity
3.84 – 3.89, typical of andradite garnets.

Demantoid Garnet is famous for its green hues:

Yellow-green
Bright grass green
Intense emerald green

The rarest stones exhibit pure vivid green with no brown or yellow tint.

Demantoid Garnet has a Mohs hardness of 6.5–7.0, suitable for rings, pendants, and earrings.

It is tough for garnets but can cleave along inclusions; proper setting protects it from chipping.

Most demantoid garnets are untreated. Occasionally, heat treatment may be applied to slightly improve colour, but this is rare. Fine-quality stones are natural with no coatings or fillings.

Avoid harsh chemicals and ultrasonic cleaning.
Use warm water, mild soap, and a soft brush.
Store demantoid garnet separately from harder stones.

Major sources of demantoid garnet include:

Russia (Ural Mountains)
Namibia
Italy
Madagascar