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This unique 4 sided die is easier to roll than the standard tetrahedron and it's easier to read the result.
The shape is a gyrobifastigium, an order 2 polyisohedron. It has 4 square sides and 4 equilateral triangle sides, though only the square sides are stable.
For more information on unique dice shapes, see http://loki3.com/poly/fair-dice.html.