So, I've got some soul gems, included some filled black soulgems. Enchanting skill 75, perks to increase skill related enchants and 4/5 on just base enchant perks.

I want to create an amulet of destruction.

3 of my soul gems (black, azura's star black, and a greater) all give me 7 or 8% reduced spell cost.

My grand soul gem gives me 20% reduced cost.


What did I kill?

I can only speculate that the souls you have in the Black, Azura's and Greater, are all significantly less than a Grand sized soul. I've definitely accidentally filled a black soul gem with a lesser soul before, though not sure how.

Azura's Black Star will only hold Grand Souls.

Wrong. Even though it says it can only hold "Black" souls, it can also hold "White" souls, making it superior to the normal Azura's Star. Every Humanoid NPC in the game (Playable races) has a Black Soul, which is automatically Grand. One shotting a Lvl 6 Bandit for a Grand Soul is fun.
I use the Mace of Molag Bal along with the Black Star to act like how Umbra+Azura's Star did in Oblivion. Not to mention, immediately recharging the Mace of Molag Bal from the Black Star after each kill will level Enchanting fast, especially when clearing out a Bandit hideout.

Ah, so it's bugged too. I shoulda figured that.

Souls that you trap automatically fill the smallest gem you have in your inventory that will hold them. Example - if you soul trap a wolf (petty soul) but have nothing smaller than a grand soul gem in your inventory, that soul will fill your grand soul gem, and you've pretty much wasted that grand soul gem. If you do lots of soul trapping of lesser creatures, make sure you have plenty of petty, lesser, etc. gems in your inventory.

Not to worry. You can clear filled soul gems by dropping them. So a Greater Soul Gem filled with a lesser soul, ferinstance, can be dropped and picked up empty, ready to be filled with a Greater Soul.

Yeah. Dropping soul gems on the ground and picking them back up clears them, if you've accidentally stuffed a Skeever in your Black Soul gem.