Transparent colorless quartzite quartz containing tiny platelets of graphite.  When oriented properly, this material has a glittering optical effect known as aventurescence.  This stone was originally cut in India.  I have re-cut this stone by hand here in the USA to properly re-orient the optical effect and re-polished to a smoother, higher glossy finish.  Flat, semi-polished back with a beveled edge for ease of setting. You will receive the exact gemstone shown in photos.

Weight: 16.15 carats
Length: 20.47 mm
Width: 16.00 mm
Height: 6.67 mm

Gem Species: Quartz
—Variety: Quartzite
Phenomenon: Aventurescence
Trade Name(s): Midnight Quartz, Black Sunstone*, Grey Aventurine*
Origin: India
Enhancement Code(s): (N) Natural

----------------------------------------------
* Using the terms "Black Sunstone" and "Grey Aventurine" to describe this material is deceptive and a punishable crime under US Federal Trade Commission regulations! Sunstone is a feldspar containing either copper platelets or thin hematite ribbons.  Aventurine Quartz is strictly green, as a result of the chromium-rich mica platelets within a quartz host.
----------------------------------------------

American Gem Trade Association Gemstone Enhancement Codes

Enhancement: Any treatment process other than cutting and polishing that improves the appearance (color/clarity/phenomena), durability, or availability of a gemstone.

(N) Natural
No modification; natural stones which are not currently known to be enhanced (or currently has no known modification process).