Demantoid Garnet Buying Guide
Stunning green hues and a brilliance that outshines diamond combine to make demantoids truly captivating. This variety of garnet is an entirely different stone than the more familiar dark red gems of the garnet family. In fact, demantoid garnets are much rarer than the red varieties and are the most valuable type. Although named demantoid, or “diamond-like,” these garnets actually exceed diamonds in both brilliance and dispersion. Their horsetail inclusions are a delight to collectors, too.
Before your next demantoid garnet buying trip, read up on what makes this variety of the January birthstone unique and the quality factors that most impact cost.
Demantoid Garnet Buying and the Four Cs
Demantoid is the green variety of andradite garnet, with colors ranging from deep forest greens to light, nearly colorless hues. For this variety, the connoisseur’s most important consideration is the trade-off between depth of color and visible brilliance.
While demantoid has a higher dispersion than diamond, darker tones may obscure this effect. Still, medium-dark tones hold the highest value, where the green color is strongest. For those who prefer greater dispersion, a slightly lighter gem will sparkle strongly without much loss of color.
Although high clarity is always desirable, some connoisseurs and mineral enthusiasts will delight in the horsetails frequently present in demantoid garnets. These fibrous inclusions are usually yellow or golden in color and emanate from the center of the stone, curving outward. When present, a horsetail inclusion is a diagnostic indicator of demantoid garnet. However, not all demantoid garnets will display a horsetail.
Overall, clarity doesn’t have a high impact on demantoid pricing. Both eye-clean gems and gems with prominent horsetail inclusions will command slightly higher prices than gems with other inclusions.
Garnets are available in nearly every cut. Still, demantoid’s high dispersion means the gem typically holds the most value in a round brilliant cut. Small demantoids should always have good cuts with the correct proportions. Lapidaries may cut larger specimens to retain more weight.
0.85-ct stone with thin cuts to create reflective lines. © Ildar Latypov Minerals. Used with permission.
Like other green varieties of garnet, demantoid is rare in any size and especially rare in large sizes. Stones under one carat are readily available, but any gem above 3 carats is a collector’s item. Thus, price per carat increases rapidly for this gem.
Sources of Demantoid Garnet
Demantoid garnets were discovered in 1853 in the Ural region of Russia, and the Ural Mountains remain an important source for demantoid. Some collectors will pay a premium for stones from this locale. Other sources include Pakistan, Sri Lanka, Italy, Namibia, and Madagascar.
At 6.5 on the Mohs scale, demantoids are fairly resistant to scratching, but dust may still scratch this stone. Still, with no cleavage, demantoid garnets are less prone to breaking than other jewelry stones. Thus, demantoid’s high durability makes it a great gem for any type of setting.
Demantoid Garnet Buying and Heat Treatment
Low-temperature heat treatment under a reducing atmosphere lightens the tone of some very dark specimens and may remove brown hues. This treatment mimics natural heating that some deposits have already undergone. It produces a stable color. Laboratories cannot reliably detect this treatment, but some may note whether there is evidence of heat treatment. Because of the resulting color’s stability, this treatment is largely accepted and doesn’t heavily impact price.