What's the Name of That Book??? discussion

The genre was Fantasy, I remember it being fairly long. The cover I believe had a stone on the cover with a swirl on it.

The story was about a girl who gets a magical stone. The stones are the only source of magic in the world and the powerful ones are owned by the richest of the world. They create a sort of scarred tattoo on the owners of the stone, and can draw power from it.

The main character I think is named Jenna, and if I remember right she had the ability to shape shift into a seal or dolphin, I can't remember which.

It overall had a Irish feel about it, and I can't remember the name of it to save my life.

Searching "fantasy novel" "stones" "magic" "jenna" this book came up: Holder of Lightning by S.L. Farrell (Book 1 in the Cloudmages Series). A lot of your description fits but not the cover. Not sure about the shape shifting either...

That's it! Thank you!