There really isn't 'quality' to them. They basically range in
different sizes, from something to Grand Gem.
The Grand Gem's can hold up to anything of 80,000 capacity,
which are your Golden Saints, and one or two other creatures
(I haven't played this since last June :P), or you can get a
special one that holds up to 4 million, I believe.
I don't believe I ever really used mine, I know I just sold them.
And for the person asking about Dagoth Ur, are you actually
in his keep, is so, I dunno what's wrong, but if
you're outside, in the mountain canyon lava-pit
thing, there should be a lever to the right of the
door (to the right when facing the door),
btw, if you see that guy walking around in
all Glass-Armor, kill him
(And if anyone has any really good soul-gem
enchanted weapon combos, PM me, I haven't
found anything better than Sunder.)