Star sapphire

Star Sapphire was a very rare form of sapphire that exhibited asterism.

Description
Asterism is a reflective optical effect that highlights a white six-pointed star on the surface of cabochon-cut stones. Star sapphires were more opaque than regular gem-quality sapphires. About one in a thousand sapphires was a star sapphire. A typical specimen had a base value of 5,000 gp or 1,000 gp depending on market conditions.

Powers
Star sappires were often used in making and decorating devices and objects to ward off hostile magic, such as globes of invulnerability, allowing them to be effective against stronger spells. Many spells could be enhanced by adding a star sapphire to the material component. For example, the duration of an antimagic shell could be extended for more than two hours beyond the usual time limit.