I am trying to find out the name of a cartoon I watched growing up in the 70s or 80s, where a little boy goes underground and learns about rock formation by meeting singing and talking igneous, sedimentry and metamorphic rocks.

Oh my! I just suggested this film to someone else!

Dig (1972 short)
From IMDb review: "Dig" is a great cartoon about a rock that gets picked up by a kid on a bike and helps him travel into the earth to learn about different types of rocks. There's music which is both entertaining and hilarious.

Dig (1972) - IMDb


You can watch it on YouTube: https://www.youtube.com/watch?v=d6QIDwhVMkc