Mana Crystals are a blue-colored, magical mineral found inside dungeons. Although they aren't worth as much as essence stones due to not containing as much mana, they can still sell at high prices and serve as a secondary source of income for hunters around the world.
As machines that run on electricity don't work inside dungeons, hunters are required to mine mana crystals by hand. As a result, guilds typically have entire teams of hunters dedicated to just mining crystals. Since crystals are typically found in large deposits and the hunters on the teams are usually low-ranking, it typically takes hours before a dungeon is mined clean of crystals.