[Galendor]

So the consensus seems to be that invisibility was not an intended power of the One Ring, but rather a side effect of its true power - allowing the wearer into the wraith or "spirit" dimension. Mortal beings such as men, hobbits, and dwarfs would uncontrollably become invisible as a side effect of wearing the Ring, but certain beings such as Sauron, high Elves, and Gandalf would probably be able to control the shift into the wraith dimension and thus would not necessarily be invisible if wearing the Ring.

The Ring also confers unnaturally long life to the mortal wearer, I wonder if this is also just a side effect rather than an intended power. Perhaps a mortal stops ageing whenever in the wraith dimension (whenever wearing the Ring). I suppose just keeping the Ring in your pocket or on a chain would not confer long life. Gollum, I suppose, probably wore the ring for very long periods when living under the Misty Mountains. He probably wore it while sleeping.

Any light emitted by the wearer or an object they were holding would be visible.

Also I think the mortal wearer is not 100% invisible but very close to it - In The Hobbit, Bilbo found that when wearing the Ring in full sunlight, he still cast a faint shadow. So the wearer must absorb some light and thus not be completely transparent (invisible), agreed?