Onyx is a black and white semi-precious stone. Though valued by some merely for its beauty, it is also an important component required for spells and rituals that allow their casters to command the undead. As the onyx is consumed in such spells, there is a constant demand for the material by those who would seek to have the dead do their bidding.
Onyx is found around the world, but generally only in small quantities. To fulfill the sometimes vast amounts they require, many necromancers turn to trading or purchasing it, often surreptitiously, from merchants who transport the material across the Hundred Days Road from Rovakhit. The onyx mines of Rovakhit are by far the largest in the world, and the nation has become very wealthy as a result of its trade, even if the practice of necromancy is strongly opposed by those around Rovakhit.
In some places, most notably amongst the undead upper classes of Tarkuus, onyx is incorporated into jewelry worn as status symbols.