This 12 sided die is an interesting variation on the standard dodecahedral dice.
It's an octahedral pentagonal dodecahedra, an isohedron. It's also known as a pyritohedron, since it's one of the crystal forms of pyrite. This is just one example of the infinite number of pyritohedra. They typically look like a slightly warped version of a regular dodecahedron.
For more information on unique dice shapes, see http://loki3.com/poly/fair-dice.html.