That would be a chip. Below is a link to a thread with some pictures of pontils. To the best of my knowledge, your marble is "modern", that is, made after 1970. It's machine-made. There are some older marbles with embedded frit. However, most will be modern, made in Mexico or China. With some new varieties made in special runs in smallish quanties by Jabo and Dave McCullough within the last ten years.