Ok Crystal Balls! Now had a look at the stratics archive and no real clear info there so had a dig around the Usenet archives and had some better luck. Due to the lack of info I've included slightly out of era posts purely for reference and to show a continuity of drop rates/functions across the time periods.
Personally I think Psilo is correct on his claims that bog standard Crystal Balls were indeed common, and you could cast on them. I however didn't find any mention of them being available from mage shops, just that you could give them to mage vendors for 1 point of Karma. However the rarity of the singing, 8-ball or glowing Crystal Balls all seems to be fairly high due to them being token/seasonal event rewards or rare spawns. I also remember seeing regular Crystal Balls dropping from Liches back in T2A but my memory alone shouldn't be trusted.
In-Era Posts:
March 2000
http://groups.google.com/group/alt.game ... 6d8fa91a70
"Are Crystal Balls important? Can you sell them? They are everywhere
and I still do not know what to do with them. Any help available on
this one? " <-- common!
"Plain old crystal balls are useless, as best I can determine. You can
find them laying around all over the place, and the Vesper bank always
has some laying outside." <-- really common?
"I think you should be able to cast spell and target a crystal ball. Then
you can double click on the ball later and cast the spell without using
regs. Of course you would still have to recite the spell and go through the
movements. This is kind of like inscribing a scroll... heh"
Slightly out of Era Posts (for further reference):
Posts from April-May 99
http://groups.google.com/group/alt.game ... 1e76d1bb3d
"You can use them to practice spells , simply target the crystal ball with a
spell like Corp Por ! "
"Chrytal balls, when given to mages in shops give you 1 point of karma, the
higher u are in karma, you wont even get a message, but yes they give u karma. "
Posts from May 98.
http://groups.google.com/group/alt.game ... 609ddc3cb0
"Crystal balls are used to show the last 5 lines spoken. They do not all
work though. "
"Actually crystal balls can be quit usefull to those practicing magery. You
can set them on the ground in front of you and cast offensive spells into
them. The spells get absorbed and your magery goes higher. No PC, or
animals, or monsters needed."
Posts from April 98.
http://groups.google.com/group/alt.game ... 549ee85fd4
"I recently found a crystal ball on a ettin, what does it do?
Is it only for decoration, or can you actually use it for something?"
Posts from Feb-March 99
http://groups.google.com/group/rec.game ... d29c9c000c
"2. From spell casting monsters mainly, usually on the stronger ones liches and
such. Sometimes in chests
3. Use them to pratice magery on. It still uses mana and reagents. No bonus for
using them . Just lets you cast on it for gaining skill. "
Posts from Nov 2001
http://groups.google.com/group/alt.game ... 18f4f47138
"Besides, back on topic: Why? They are one of the most common loot items and
a waste of time. The only use i have found is for some house decoration."
"Why buy? Just go mug a lich on the road from Britan to Yew

"
If anyone else wants to do some further digging here is the link to the search I used
http://groups.google.com/group/alt.game ... ll&start=0&
Edit: put some posts in wrong section there.. sorry, eyes getting fuzzy reading all these.