Amaratha

Amaratha, also known as shieldstone, was a rare jewel believed to be unique to Faerûn. It occurred as nodules on old rock strata&mdash;canyon walls on the surface exposed by time, or below in the Underdark&mdash;and often in groups of a dozen or more.

Description
When rounded and polished into spheres, shieldstone was a sparkling pale green to greenish white. This gem was soft enough that it needed to be protectively mounted and worn or placed in locations that did not expose the stone to impacts that might chip or shatter it&mdash;rings, staves, or helmets were to be avoided, but necklaces, belts, or brooches were appropriate, as circumstances dictated. A typical specimen was 1 in (25 mm) in diameter and had an original base value of 4,000 gp, but as they became more scarce the price rose to 5,000 gp,

Powers
Shieldstone got its name from its ability to attract and absorb electrical discharges within about a 10 ft (3 m) radius. Unfortunately, the gems vaporized as they did so. Minor sparks from static electricity had no noticeable effect on amaratha, but once the charge reached a dangerous threshold, like a shocking grasp or a jolt from an electric eel, these stones absorbed a fraction of the energy proportional to the diameter of the stone, i.e., a 2-inch diameter stone that absorbed half of its capacity would vaporize down to a 1-inch stone, and being exposed to the same electrical energy again would cause it to vanish entirely. Any excess electricity not neutralized was felt by the target(s) of the bolt.

Wands that provided protection from fire, cold, electricity, acid, or sonic energy were especially effective when they incorporated a shieldstone in their creation.

Trivia
Amaratha stones were considered sacred to a number of deities, including Arvoreen, Mystra (including Mystryl and Midnight ), and Yurtrus. It was appropriate to consecrate or sacrifice shieldstone to these deities, or a follower might find one as a boon or an omen.