My death pet has the death-dealer talent. If I add a death-dealer jewel, will the jewel work or will the talent cancel the jewel out?
I don't think it will allow you to add it, but if it does it will be wasted. The jewel will get a red a line through it and the stat won't be counted. We can only have one of each talent.
The reason it sometimes shows as a line through the jewel is because it's possible to train a pet after a jewel has been added. If it manifests the same talent as the jewel it crosses out the jewel so you know it's not affecting the pet any more and can be replaced with something else instead.