DEMANTOID

A beautiful example of a horsetail inclusion within a demantoid.

Variety:  Demantoid
Species:  Andradite
Group:  Garnet
Mohs Hardness:  6.5 to 7 out of 10
Color:  Green to yellow green, with strong fire
RI:  OTL
Birefringence:  None
SR/DR/AGG:  SR, often with ADR
Clarity Type:  Type II
Pleochroism:  None
Spectrum:  A dark band at 443nm in yellowish green stones, or a cutoff below 443nm with fine lines at 620nm, 632nm, 693nm, and 700nm in rich green stones
Fluorescence:  Inert
Color Filter:  Pinkish to inert
SG:  3.84 (+/- 0.03)
Routine Treatments:  None

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Demantoid Facts

Platinum eternity band set with demantoid garnets

While garnets are generally thought of as reddish gems with a darker tone, they occur naturally in many different colors.

Demantoid is a very beautiful green variety of garnet that was first discovered in the Ural Mountains of Russia.  This rare type of garnet can display rich green hues that compare to those of fine emeralds.

Some demantoids contain a type of "signature" inclusion, described as a horsetail. 
While highly-included diamonds are less valuable, demantoids with horsetail inclusions are sought after by collectors, and can be much more expensive than cleaner material.

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All photos courtesy of Scott Papper, GG, ASA, CMA, RGA, AJP, CMSP, CSSP