Rosaline

Rosaline, also known as unionite, thulite, or pink zoisite, was found in large quantities of soft stone (usually cut in 1 lb/2.2 kg blocks for trading), or in small, hard crystals.

Description
The opaque pink variety of this ornamental stone was relatively soft and could be tumbled or cut cabochon while the harder crystal form could be faceted by a lapidarist and exhibited trichroism: three colors in the gem, each viewed from a different angle. The three colors were purple, red, and either blue or green for the third color. A typical soft pink stone had a base value of 10 gp, whereas the trichroic crystal variety had a base price of 50 gp.)

Powers
Some tri-colored rosaline crystals with the proper mix of colors had the property of negating one color of any prismatic spell. A correctly attuned crystal would vanish when the creature carrying or wearing it came in contact with the first color layer of the prismatic spell effect, collapsing the layer with no harm to the creature. Negating the next layer required another crystal, and so on. Unfortunately, there was no known way to identify rosaline crystals that exhibited this property beforehand.