The powerful artifact that grants invisibility to its wearer in Tolkien’s Middle-earth is the One Ring.
Sauron forged the One Ring in the fires of Mount Doom during the Second Age. He placed much of his power into it so that he could dominate the other Rings of Power and control their users. The Ring can make an ordinary wearer invisible, but its deeper purpose is domination rather than simple concealment.
Its invisibility effect is limited and dangerous. The wearer is shifted partly into the unseen world, which can make them more visible to servants such as the Nazgûl. Prolonged possession also encourages obsession and corruption, as shown by the effects on Gollum, Bilbo, and Frodo.
The Silmarils are jewels from Tolkien’s earlier mythology, while the Palantíri are seeing-stones. The Arkenstone is a treasured jewel of the dwarves, not one of the Rings of Power.