Asphaltum is petroleum tar. It is a deep, wickedly dark brown color - about the color of coffee so strong it stinks. Gary Brumfield, past master of the restored gunsmith shop at Colonial Williamsburg, has been something of a champion for using asphaltum as a stain for curly maple. The traditional staining technique is of course dilute nitric acid, with iron mixed in, making a horrid poisonous reddish mud that you paint on the stock, heat with something like a bar of iron heated red hot, at which point the acid soaked wood turns a glorious reddish brown. The curl in the wood has great clarity and vibrancy (the actual word for this dancing curly maple figure is "chatoyancy", I kid you not...) and it's well documented as being the standard staining method time and again in period sources.
But there are other ways. I am going to research the documentation on asphaltum, but in this simple photo you can see the piece of maple scrap really jumps out. The color is more brown than the red-brown typically seen with nitric, but then I've gotten this same color before using acid, so it's a bit of a crap shoot. Part of the fun.
Although asphaltum will dry hard, I will follow tradition and use a beeswax finish on top. This makes a really tough, waterproof finish.
Just a quick pic - I'll post more when I finish the rifle. Notice there is no visible curl in the wood because the sun is bleaching it out, but the asphaltum really makes it stand out.