Prism

A light-splitting glass, priced at 10 G.P.

In an alchemist's hands a prism detects illusions. The refracted light must be studied for a full minute, then a save vs Intelligence shows whether an illusion is revealed. Only alchemists can use prisms this way. They make them from 1 G.P. of silica, potash, lime and lead plus a dram of rare earths, cast into a many-sided crystal and finely polished.