Recently given two new Studding items in the update on the tenth.

"Two more items have been added to the ‘stud item pool’, where you can send items over in a stud request to be used on the stud you’re requesting at the point of breeding. These two items are: Opal Saltlick and Rock Salt."

They're found in Monkey Business for 200 for a Rock Salt and 500 for Opal Saltlick. Respectively because the plain rock salt gives choice of parent's skin while opal saltlick gives choice of eye color.

Rock Salt: Give this mineral to your chosen lion/stud to make their next litter inherit the skin colour from the parent you chose.
Sell for roughly 30

Opal Saltlick:Give this mineral to your chosen lion/stud to make their next litter inherit the eye colour from the parent you chose.
Sell for roughly 75

One thing I know I was personally unsure of: How to use it.

Both need to be given to your lion BEFORE breeding. Checking the box on stud requests doesn't allow you or the stud owner to choose. It has to be consumed before breeding which in a stud request is reduntant.

By checking the box to send the Rock or Opal salt in the stud request it vanishes. Neither have the item it seemingly vanishes once the breeding is accepted. The stud wasn't able to use it and neither was the lioness.

From current uses of the new studding items, Opal Saltlick will be the better choice as the Rock gives skin choice rather than base:

So if you breed Anjeer (White Skin) to a Tawny (Brown Skin), you get to choose the litters' skin type from White or Brown.

Where as using the Opal Saltlick:
You breed Starshine to Jasper, you get to choose the litters' eyes from Starshine and Jasper. Which is often seen more than actual skin.

Also important to note that to choose the skin or eyes, you have to use the item on the lion you'd like their genetics to be passed on. So if you want dad's eyes to be passed on to the litter give him the salt lick, same if you want mom's eyes to be passed on.