Silex

Silex is an item that allows players to add up to 5 extra levels of  to Pickaxes or Drills through an anvil (1 per use). Legacy version of this item (crafted with 9 Nether Quartz and have the texture Enchanted Quartz) can only be obtained through Players Trading and Auction House.

Sharp Rock
After the Crystal Hollows (0.12) update, the original Silex that was "released" for a day were changed to being called "Sharp Rock"s.

Trivia

 * In French, "Silex" directly translates to "Flint", the current texture of Silex.
 * This item was originally "released" for a day accidentally, alongside the Flamebreaker Armor.