Greenstone

Greenstone was the Faerûnian name for chlorastrolite, a gray-green variety of pumpellyite.

Description
These ornamental stones were soft and usually cut cabochon or tumbled and polished. High quality greenstones could take on a glassy finish and such stones were sometimes called chlorastras. Greenstones were found in solidified lava flows, in nodules of up to three quarters of an inch (two centimeters) in diameter. Exceptionally large specimens were made into greenstone amulets. Greenstones were plentiful and easy to work with so a fair amount of fraudulent amulets were created and enchanted with Nystul's magic aura. A typical stone had a base value of 10 gp.

Powers
Flawed stones and those too small to be useful were crushed to powder and used as a material component in casting abjuration-type spells that protected the mind. However, certain spells were ruined when this gem was added, making them completely ineffectual.

This gemstone was one of the "nine secrets"&mdash;minerals that could be enchanted to make an ioun stone.