I have the option of disenchanting a ring that adds +20 to health but cannot disenchant an amulet that adds +10 to health. The only difference in the two is the amulet is stolen so am I correct to assume you cannot disenchant stolen items? Thanks

That may be right, I've had a few items I couldn't enchant and wasn't sure why. Doesn't make a lot of sense though, if you sell it to a fence and then purchase again, is it still counted as stolen?

Does it not show up on the list or is it greyed? If you already disenchanted the +20, the +10 is the same enchant. There's also quest items, artifacts, and a number of other unique pieces that can't be disenchanted.

No it doesn't show up on the list at all, the ring does but the amulet does not. It might be a quest item though