Iris agate

Iris Agate was a low-grade agate that came in large pieces, usually sold by the pound (kilogram) as a building material.

Description
Iris agate looked white in normal lighting conditions, but when cut into thin slabs and illuminated from behind, its swirling mix of colors was strikingly visible. Iris agate was often used in temples for this effect. A typical price for large quantities of this hardstone was 2 gp per pound (0.45 kg).

Powers
A sufficiently large slab, screen, or statue made of iris agate reflected all spells cast upon it back at the caster. Treasure vault doors or other areas that might come under magical attack were sometimes lined with iris agate or had iris agate statues strategically placed for defense.