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.
Note that this is a blank, so the faces will need to be numbered. For more information on unique dice shapes, see http://loki3.com/poly/fair-dice.html
1.84 w x
1.84 d x
0.724 w x
0.724 d x