Probably some sort of ABS... But don't quote me haha.
Fun Fact: "Clay Dots" made famous by Fender in the early 60's were never made up of Clay. The material used was "Vulcanized Fiberboard" that were inlayed White as a ghost. They would turn that yellowish color as they aged and wore. They also contained cellulose like the old pickguards. The test of time didn't work out so well as they shrunk and discolored. I'm still a huge fan of the color resembling "Clay Dots", adds a touch of elegance to a dark wood board