About the only thing you can ‘mine’ out of lava is semi-precious OLIVINE crystals… a light green quartz type crystal. Don’t ask me the chemical reaction, but it’s one of the first things to crystallize out of hot lava under certain cooling conditions. I’ve seen lots and lots of lava rock with embedded olivine crystals.
Some of the first sailors to ‘discover’ Hawaii at Honolulu Harbor found an old shoreline crater rim full of crystals and thought they were rich! They named the place “Diamond Head”. Today it is a landmark next to Waikiki Beach.