I see your confusion… its 2x or 3x the drop rate not double or triple the stones.
Base Quest/Challenges/PvP etc. Rough Numbers: Minor 60%, Major 30%, Runic 9%, Arcane 2%, Celestial 1%. Before you start an Explore you’ll see at the bottom Runic, Arcane, and Celestial have an increased percentage to drop not double (or triple) stones. So sayaround: 18%, 6%, 3%