Serpentine

Serpentine was a combustible black powder known to the Istari or wizards. Gandalf the Grey used it in his magical fireworks, which he would blow for birthdays in the Shire, such as those of Old Took and of Bilbo Baggins.
During War of the Ring, the head of the order, Saruman, hit upon weaponizing the powder, which he deployed against the Wall of Helm's Deep to breach it, and very nearly won the Battle of Helm's Deep.