Obsidian can be mined fast-ish with non diamond picks

All non-diamond picks will mine obsidian faster than with no tool although every tool except diamond is supposed to be 4m10s,

most notable are gold, and iron.
Diamond pick can mine obsidian in 9.5 seconds
Gold pick can mine obsidian in 19.5 seconds
Iron pick.^ can mine obsidian in 38 seconds

Time is slightly rounded