Yea, clean it first. Can’t tell you how many black stones, I have bought hoping they were arks, only to find out they were something else. Fortunately for just a couple bucks.

As said start with a good hosing of oven cleaner, wait a few minutes and hose it again and scrub. Then wash in soapy water, that should at least reveal the color and any pattern.

If you want to get all the oil off it, you must soak in water and degreaser, I use Simple Green and or, Awesome from the dollar store. Some hard stones can take months to release all the oil.