Just wanna sanity-check a couple of things, because this looks really cool, but there's some ambiguities in the wording; also, what it appears to do strikes me as awesome enough to be a little disturbing.
It appears to me that this item gives you unlimited travel to anywhere, other planes included. However, you can't take it with you when you go someplace, so it's not a very useful strategy for going places, and of course, since the portal you come in through is still there, someone else can leave through it and take your mirror. Although I'm assuming that the "trapped there" doesn't really matter much if you have other ways of travelling.
On the other hand, for "hang on, didn't bring the right wine for my iron rations" trips, it's probably pretty useful.
... Of course, it's also pretty large to be carrying around.
But... What do you think the range is? Other planes implies Pretty Far, but strictly speaking, clairvoyance doesn't have very long range. Should I assume it's also infinite range (or close enough that I don't care) on my current plane?
(Sadly, it weighs too much to be summoned with Instant Summons, so you can't use that to pick it up on another plane. But at that point, it'd be cheaper to just cast gate.)
