what magic item is this?

I honestly can't remember if this is a Pathfinder or DND item (or both), but it was something like a metal hoop that is linked to another identical hoop and functions as a small (about 2 feet across) portal. It's mainly used to send stuff long distances, but it said that a small creature can squeeze through.

Ring Gates

https://aonprd.com/MagicWondrousDisplay.aspx?FinalName=Ring%20Gates

Since Valandil Ancalime has answered the actual question, I will relate an anecdote instead:

Back in the early days of 3.5 we defeated a dragon out in the wilderness miles from anywhere. Thinking that we would only be able to transport a tiny fraction of it, he gave the dragon a truly impressive hoard - millions of gold pieces.

(Un)fortunately, he had forgotten that we had recently acquired a set of ring gates. One end was with us, and the other was was in the vault of the castle we had just acquired. A bit of shovelling and a teleport later and we were back in our new castle, several million gp richer.

TBF, the GM did not try to backpedal, he just went with it, and it did not really break anything. It lead to things that would never normally happen, like like my Wizard having a +6 Cloak of Charisma!

_
glass.

It's amusing how many stories there are about GMs making similar mistakes with unwieldy treasure.

"Floating disk! I completely forgot about that spell!"

Hah, yeah, never underestimate the power of greed to motivate the PCs to find a solution you had not imagined :)

Are they used for anything else but transport? I was thinking they could come in handy with long-distance relationships, if you get my drift (Ok, I can't be the only one who thought of that!)

Ring Gates

Linkified.

Hah, yeah, never underestimate the power of greed to motivate the PCs to find a solution you had not imagined :)

Reminds me of the story of a party encountering mithral cages holding monsters. They killed the critters then worked on collecting the cages.

Another story: party encounters a solid platinum door. GM found out he had just given the party a huge pile of loot when they collected it.

/cevah

Once came across a massive adamanine door in a dungeon, we were level 10 I think? I spouted off the likely value of that much adamant, and my disbelief thst the builders here had spent anything close to that amount. "So I'd like to make a will save to disbieve." GM thinks for a second, and agrees. The wooden door was quickly chopped down.

I used a pair of those in an adventure I wrote, one was in a lich's lair, the other end was carried around by his minion. They villains mainly used it to pass messages. The party killed the minion and then proceeded to carry around the single ring for a long time before they got the courage to look inside. Their halfling wizard cast reduce person and hopped through it. By that point in the game, the lich had been killed a long time ago and all that was left was some low level encounter and piddly bit of treasure meant for them to have found 8 levels ago. There was also some clues about some really old plots that had been long forgotten.