Crystal Hollows

The Crystal Hollows are a mining area of tunnels full of Gemstone veins, with structures that are placed differently in every lobby.¹ You get in by buying a 6-hour pass from Gwendolyn in the Dwarven Mines once you reach Heart of the Mountain tier 4; finishing Dulin's quest gives permanent access.¹

Mining here is different. Broken blocks become air instead of bedrock, and stone is replaced by Hard Stone, which is harder to break and has its own collection.¹ Iron and gold ore drop ingots directly, no smelting needed.¹ You can place torches, ladders, Hard Stone and cobblestone.¹

Crystals and the Nucleus. The Hollows are split into five zones, each tied to a gemstone, and each zone hides a structure with a task that rewards its Gemstone Crystal:¹

Jungle Temple: bring a Jungle Key and finish the parkour.¹
Mines of Divan: find the lost parts with a Metal Detector.¹
Lost Precursor City: hand in the six robot parts.¹
Goblin Queen's Den: bring King Yolkar a goblin egg.¹
Khazad-dûm: defeat Bal.¹

Crystals are placed in the Crystal Nucleus at the centre for rewards, or used at the Forge to make Perfect Gemstones.¹ Treasure chests found while mining are the main source of Gemstone Powder.¹

A lobby stops taking new players on its 20th in-game day and shuts down by the 36th.¹