I tend to find too that quality tends to dictate rarity rather than quantity. It is very easy to get ahold of an alright Wind Wand or some tatty Mage Robes, but finding near perfect quality of such things is always a struggle.
Gems and magical weapons are not rare. It takes a few days of hunting to get ahold of any magic Gem, save the magical rubies, which I would consider somewhat rare owing to the difficulty in collecting them. 2 Gold is the standard rate for one of them. Magical weapons are dropped in high qualities more often than Gems and so I wouldnt consider them rare either. I'd sell a magic Serinjah for 30-50 silvers depending on quality.
Diadems are pretty tricky to get ahold of considering there's no active goldsmiths able to make them to an acceptable quality (acceptable in the sense that, if you are looking for one, you are looking for a near perfect one). The same principle goes for near perfect Topaz rings, they only usually drop at slightly above average quality so they are quite uncommon too.
Icebirds are a dime a dozen if you've got access to a half decent group and a few Yr maps. Still, not many people do so they can go for 50 silvers upwards.
Shadowplate and Magical Elven is the same deal, there's a fair few decent ones floating about that anyone with the capital can get ahold of easily. Nightplate is quite rare, but its also rubbish so there isnt much demand.
Drow equipment is always rare too, along with flame weapons. Archmage rings go here too but I discount this class of items as they arn't obtainable without GM assistance. Dragon eggs are quest items that do nothing, so in a sense they are rare like the way a banana is rare.
As mentioned previously it is only custom items that have a sentimental value that are truly rare. The archmage ring of Samantha Meryadeles or the Cheiftain's Shadowplate for instance. Trophies and spoils of war of which there is only one.
Can I enquire as to why you ask?