[Well there are only something like 25 regular polyhedrons, if you had wanted to stick with the dice theme you could have rolled an 8-sider and a 4-sider, forms we know can exist. Either use the 4-sider to generate how many 8's to add to the 8-sider roll (0,1,2 or 3) or vice versa. Other numbers might take more effort to come up with a method.
Let me be clear I am not taking issue with using the roll dice feature of the game, I just thought you might dress it up to make it more in context. I do not care how special your dice are it is just not possible to have an object with an arbitrary number of sides be a "fair die"
The full geometric set of "uniform fair dice" (face-transitive) are:
* Platonic solids: 5 regular polyhedra: (4, 6, 8, 12, 20 sides)
* Catalan solids: 13 Archimedean duals: (12, 24, 30, 48, 60, 120 sides)
* Bipyramids: infinite set of prism duals, triangle faces: (6, 8, 10, 12, ... sides)
* Trapezohedrons: infinite set of antiprism duals, kite faces: (6, 8, 10, 12, ... sides)
* Disphenoids: infinite set of tetrahedra made from congruent non-regular triangles (4 sides)
* "Rolling-pin style dice" (also called "rolling logs" [3]) are the only way to make dice with an odd number of faces[4]. They are based on an infinite set of prisms. All the (rectangular) faces they may actually land on are congruent, so they are equally fair. (The other 2 sides of the prism are rounded or capped with a pyramid, designed so that the dice never actually rests on those faces.)
Wikipedia I suppose, having looked at the quote I posted and some other links, I really have to withdraw my objection. Perhaps some day gem cutters will be able to make sets of dice for all occasions.
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