Crystallization (sp?)does not make them worthless it is a process that can be slowed and stopped. Mnay older dice will crystallize with age (DUH!)

Crystallization occurs from plastic fatigue (result of their age), moisture and ultraviolet (UV) light exposure. Crystallized dice are very brittle and they can break easily if to much pressure is applied. Crystallization begins at the corners and the dice become yellowed or amber-colored with age. One way to preserve crystallized dice is to paint their surface with nail varnish.

FROM THE DICEMANS WEB SITE!

http://www.diceman.ch/school/school_material.htm