With inspiration from SailorMidgard and the Silver Nails item. I decided to give it a go with one for the vampires.

(any suggestions are welcome and please feel free to post em!)

[Info Text]
Blood gem : Crafted by the Blood Maidens these gems are worn by vampires out of both fear and faith. By wearing a blood gem a vampire sacrifices some of its own blood and a portion of any blood it consumes to keep the promise of never letting the Red Lady run dry. In return for this offering, The Red Lady bestows her gifts. But be warned, if one is reckless and draws in contaminated blood, she will not be kind.
To have a blood gem crafted requires blood, sweat, and tears.... not yours of course.

[Bottom of card Quote]
"The blood runs so sweet, Come let us drink!"

as a Card type item it could have unlockable tiers that would first take more life to use but in return add more bonuses.
(as we don't know how stats will work in the game I am leaving it simple with %)
Tier 1 - 1% life lost per (x time - for now say 1 minute) / 5% blood consumed given to lady / 1 bonus at 5%
Tier 2 - 2% life lost per (x time) / 10% blood consumed given to lady / 2 bonuses at 10%
Tier 3 - 3% / 15% / 3 at 15%
Tier 4 - 4% / 20% / 4 at 20%
Tier 5 - 5% / 25% / 5 at 25%

Bonuses could include things like.
attack damage
attack speed
cast speed
cast damage
magic resistance
daylight adaptation
weather/elemental resistance

To keep the gem in context with its theme. These would NOT be options
Extra life / mana / essence
More life per draw
faster mana / essence regen.

as far as a skill tree (if it were made into a card) starting at the top and working down would be an easy way to go about it.

bloodgem skill tree.png
If this was not a card but rather a one use type item like a potion of sorts then different tiers of gems would naturally cost different amounts along with what bonuses they included.
I think this is a brilliant idea and would be a fine edition to the game.