MAKE contributor Steve Lodefink posted results from his Lumenlab Micro CNC‘s maiden voyage -

Lithophanes were a popular way to hide girlie pictures in the bottom a of gentleman’s tea cup around the end of the 19th century. An image would be molded into the porcelain in the cup so that only when held up to a light would the picture be visible.
[...]
I “lithophaned” an image of a skull into a piece of corian. When viewed under normal front lighting, it sort of looks like a distorted C-3P0 face, but when held up to a light source, it is transformed into a skull.

Check out video of the ‘skullithophanery’ in process over @ Finkbuilt.