Perhaps today’s inclusions will suit you better. These polymer clay pieces (from Hollie’s collection) by Lindly Haunani are made from translucent clay. When baked, crayon shavings mixed into the plain clay leave their color and hollow spaces behind. Generally, 1⁄2 tsp. of chopped crayon per ounce of translucent clay is a good ratio.
Lindly published a great tutorial about inclusions long ago (1999) in Bead & Button. It’s very inspiring and luckily she keeps the information on her web site for everyone to enjoy.