Raindrop

Raindrop is the name given to the purest form of cassiterite, specifically to flawless, colorless crystals or colorless areas inside larger, dark brown cassiterite crystals.

Description
These stones, in their most prized form, were clear, hard, and durable, suitable for small faceted gems or they took a velvety smooth polish. Often they would be cut and polished into a teardrop shape and when placed on cloaks and other garments resembled raindrops, which gave them their popular name. The colored part of the stone was much less valuable and known as woodtine. A typical specimen had a base value of 500 gp.

Powers
This gemstone and its woodtine form could be used as "residual magic" detectors, more sensitive than detect magic and other similar spells. When either variety was brought into contact with another gem, stone, or metal, it would darken in color if that item had been previously enchanted and had since released its magical energies. The color change was temporary and repeatable.