Silicone mat with a crystal structure. The size of the mat is 40 x 52 cm (15 3/4" x 20 1/2") and it is very thin and flexible. The mat can be used to cut your own inlay molds, in any shape that you like. You can then use them in other silicone molds, even against a vertical surface.
And because it's so thin, it's very easy to cut with for instance a hobby knife, scissors or even with an electronic cutting machine like a Cricut or Cameo.