A 50-sided die in the shape of a prism-expanded dissected cuboctahedron, or equivalently an expanded dissected rhombic dodecahedron. This shape has 14 faces from a cuboctahedron, 12 faces from a rhombic dodecahedron (the dual of the cuboctahedron), and 24 faces from the edges of these dual polyhedra, for a total of 50 faces.
8 faces are equilateral triangles, 12 are 1:sqrt(2) rhombuses, and 30 are squares. Because the faces have different shapes, this is polyhedron does not make for perfectly fair 50-sided dice, so label the smaller triangular faces thoughtfully.
I also sell another pentacontahedron (50-sided polyhedron) with a different arrangement of faces.