They can also randomly be obtained via Login Chests that can be claimed ca. every 4 hours for free. During even-times, the content of Login Chests might differ and Arcstones can temporarily become rarer.
While common blue Keepas usually only yield 1-5 Arcstones at a time either as a loot or pet-harvest, Silver Keepas can yield up to 30 Arcstones (only when killed), and Golden Keepas up to 50 Arcstones (also only when killed), perhaps rarely even more.
All types of Keepas can spawn literally everywhere, even on crafted blocks. But they spawn most often on the Corruption layer, so you might want to hunt for them there for maximum efficiency. Best take some Corrupted Food with you that will turn Corruption damage into healing over time. A few Healing Beacons and some ladders should also be very helpful.
When a Keepa of any kind is near, a symbol that resembles an Arcstone will start to blink and "pulsate" with blue circles coming from it on your compass on top of the screen.
If you tame Silver Keepas or Golden Keepas, feed them their favorite Food and harvest from them, you can usually only obtain 1-5 Arcstones with each harvest, different from their loot that they will drop when they are hunted and killed. You can receive the same amount of Arcstones from common blue Keepas too. Only the additional type of harvest differs.
Please note that all Pets will always only provide you with their best harvest after you have fed them their exact favorite Food. You might not get any Arcstones from any type of Keepa otherwise. For example: if your Keepa pet prefers Sandwiches (as shown in their pet window), but you feed them Turnip Sandwiches, its pet-harvest might only consist of coal and gunpowder.
Arcstones are required to craft Lumite Armguards, Lumite Breastplates, Lumite Leggings, Lumite Pauldrons, Lumite Swords, Arctek Chests, Grand Arctek Chests, Arctek Torches, Grand Arctek Torches, Arctek Lanterns, Grand Arctek Lanterns, Arctek Tables, Grand Arctek Tables, Delay Gates, Inverter Gates, Logic Gates, Number Comparison Gates, Galactic Flux Incapacitators, Block Phasers, Arc Signs, Small Capture Blocks, Medium Capture Blocks, Large Capture Blocks, Arc Picture Frames, Checkpoints, Adventure Gates, Sensors, Mob Spawners and Loot Spawners.
Like many types of animal loot, Arcstones cannot be placed nor dragged into a quickslot nor directly into the world. However they are special in that they can be put on display in Wood Planters and Flower Pots, on Wall Shelves, Hidden Temple Altars and Placemats, as well as on Galactic Grav-Chambers, Snow Buried Containers and Frozen Containers.
In former versions of Creativerse Arcstones could also be very rarely found in randomly spawning Wood Treasure Chests at night on the surface, but this is not the case any longer).