And yet, it mentions in the Expanded Psionics Handbook that Druids can't wear Crystal armour.
Seems really silly to me. In my home game I justified it by ruling that Druids and Rangers could only wear armour that at some point had been alive (ie, all the PHB armour they can wear), otherwise they suffered from Telluric spell failure (equal to ASF chance +20) as their armour was unable to channel the Ley energy that they tap into to cast spells.
So none of that stupid Stone armour from Races of Stone either, but I also introduced Truemetal, which you can occaisonaly dig out of really big Earth Elementals (as in Elder, Monoliths and Primal Elementals).