So, I get some of my casting stuff from a company called Smooth-On. Their molding silicone is not a good as Tap Plastics, but it's way cheaper so it's good for bulky things without fine details... sorry, that's a tangent.
Anyway, I am on their mailing list, and they have an interesting new product that I think is relevant to our interests -
Quarry Tone Granit FX Resin Filler.
It goes for about $15/quart, and their resin is $26/approx a quart mixed (about the same as Tap's).
I've attached a screen. I think it would be pretty neat for making rocks out of those model railroad molds I see at Hobby Lobby sometimes.
I hope this wasn't too spammy, I just thought this was neat looking.