AA is volcanic rock. Lining hollowed out leaves LG.
I've solved all the clues but am at a loss to understand the three small numbers. Also the pairs of letters make little sense unless a town in Arizona is involved.
By carefully considering all of the posts this week, I have finally completed the grid and the requisite instructions. Although I have a mathematical background, I was not familiar with the originator. Thank you to all contributors and hats off to the setter!