Five verified tiers, one documented rule
Stone, Gold, Iron, Diamond and Purple are real pickaxe names attached to the game by Roblox badges. The official description supplies the rule that newer pickaxes give more Power; nothing public supplies their exact numbers.
The upgrade ladder is the backbone of the whole game: swings build Power, levels pay Wins, and Wins buy the next pickaxe, which raises every future swing. Roblox's badge statistics turn that ladder into measurable reality — 22.06 million players hold the Stone Pickaxe badge and 11.58 million hold Purple, a 44% reach rate that makes the full ladder a normal outcome of playing. Community showcase videos ("noob with the BEST pickaxe" restarts, Desert-world speedruns) all treat the newest unlock as the strongest tool available, which matches the description's wording. What nobody has — developer included, as far as any public source shows — is a published multiplier, price or per-tier Power value. When your server shows a requirement, that number is the most current price in existence; treat copied price tables from other sites with suspicion.