Its main use is as a crafting material for items in the Ancient Tab. It can also be used to repair a Broken Ancient Pseudoscience Station, opening up more crafting options.
Although Green Gems are somewhat rare, it is possible to use a Construction Amulet to craft 5 Thulecite Suits for 15 Thulecite, then use a Deconstruction Staff on those 5 Thulecite Suits to get 30 Thulecite back for a profit of 13 Thulecite (the Amulet costs 2 Thulecite). This practice makes it possible to collect big amounts of Thulecite from a few Green Gems.
Slurtles, Snurtles and Rock Lobsters will attempt to eat dropped Thulecite, so players should be careful and collect all Thulecite before they successfully eat it. Slurtles will also attack a player holding Thulecite if there aren't any other minerals around for them to eat.
When a Slurtle or Snurtle has eaten 2 pieces of Thulecite they will produce Slurtle Slime. When players feed Thulecite to a Rock Lobster, they will become friendly and follow the player around for 3 minutes (6 Clock segments).
If you need to turn Thulecite into fragments, craft walls and hammer them. 1 Thulecite becomes 6 walls which yield 1 fragment each, making this process completely reversable at the cost of the hammer wear.
Trivia
Thulecite may have been based off of real-life mineral thulite, which is also a variety of zoisite.
Like much of the Ruins, Thulecite was inspired by the Cthulhu Mythos, and may have derived its name from the name of Cthulhu, an ancient god-like being associated with insanity, magic, abnormal geometry, and tentacles.
A secondary inspiration for the name may have come from Thule, a northern region in ancient geography that is usually described as being the farthest north one can travel. Later, writers would use the term Ultima Thule to describe something far off and unattainable. This is more than likely inspiration for Thulecite's status as an endgame material.
The material itself is quite similar in color and use to the Dwarven Metal that is found in the The Elder Scrolls series, as both the Dwemer and the Ancient Civilization used their own material (Dwarven metal for the Dwemer, and Thulecite for the Ancients), and when they became extinct, their respective material were left behind and could be used by those who found their ruins. This is perhaps unsurprising, as it's a believable explanation for the rarity of the resources required for rare elite items.
Attention: The following categories are only for the players look up the page based on their first impression, they have nothing to do with the actual categories/behavior/function/tags of these items