gilsonite

I would strongly recommend against its use in artwork intended for permanence, especially never mixed with oil paint. The bitumen never dries, will remain sensitive to higher temperatures, and will shrink and likely alligator over time especially if used with, over oil paint.
It is up to you if the effects that it affords is worth the risk if intended for long-term decoration. Probably this is less of an issue for house decoration, however, it will be difficult to remove or overpaint down the road when/if you intend on redecorating. There are a few “asphaltum hue” artists oil paints composed of modern organic pigments, and not bitumen, that may be a better choice. I remember using one in a faux finish that was rather convincing.