Aventurescence

In gemology, aventurescence (sometimes called aventurization) is an optical reflectance effect seen in certain gems.

The effect amounts to a metallic glitter, arising from minute, preferentially oriented mineral platelets within the material.

These platelets are so numerous that they also influence the material's body colour.

The words aventurine and aventurescence derive from the Italian "a ventura", meaning "by chance".

This is an allusion to the chance discovery of aventurine glass (goldstone) at some point in the 18th century.