Opal
Opals are the most valuable gems in the world for both their good and evil uses. Although an opal is not dangerous in itself, it is one of the key components of the ichor machine, which allows an individual to surpass the ordinary limits to their magic. Thus, opals are highly prized, and many conflicts have started over their acquisition.
Opals have a higher water content than other gems, and this is perhaps the reason why they are considered to have unique magical properties for humans. They are consistently valued across human cultures past and present. Some humans (especially more experienced magi) have a "sense" for opals and find them oddly enchanting. However, their beauty, value, and technical applications make them important for everyone.