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From Terraria Wiki
The Mythril Pickaxe is a Hardmode pickaxe, and is the pickaxe alternative to the Mythril Drill. This tool can be used for those that prefer to use pickaxes, or for those who find the drill's noise to be annoying. Appearances and sounds aside, the two tools are functionally identical (except for that the Mythril Pickaxe inflicts knockback, whereas the Mythril Drill does not). However, unlike drills, pickaxes are reforged with any melee modifiers, including size, speed, and knockback.
It can mine any block except Chlorophyte Ore and Lihzahrd Bricks.
The best Modifier is Legendary. While Light may seem to provide a better speed boost by percentages, after rounding Legendary and Light mine at equal speeds.