These rare stones have the ability to turn the holder into a form similar to a shadow, (as Monster Manual).
Shadow Form (AC 7; MV 12”; HD as per character; hp as per character; #AT 1; D 2-5; SD Magic weapon to hit, limited spell immunity)
Characters in shadow form are immune to the same attack forms as shadows. They cannot be turned as they are not undead, and they cannot drain strength. They are 90% undetectable, except in the presence of bright light. It is possible to fit under doors and through cracks while in shadow form. The effect lasts 2d4 turns and each stone can be used 1d4+1 times. After its final use, the stone turns into a piece of black opal (base 1000 gold crescent value).