Mithril is an incredibly rare metal. It is quite lightweight, completely non-reactive to common elements, takes an exceedingly good edge and holds it almost indefinitely. However the property which makes it truly valuable is that it is the most enchantable of all metals. Mithril is known to be able to withstand and channel the most powerful runes and hold more of them effectively than any other material the dwarves have yet discovered for the purpose.
The most powerful artefacts therefore are invariably made of mithril or have a mithril component. Mithril has an exceedingly high melting point for a metal and the forging of it is (as might be expected) very difficult.
Its value is at least that of 100pp for an unwrought coin-sized piece. Typically mithril is not wasted on actual coins.