Pyrophoric Metals - Finely dispersed powders of magnesium, zinc, or thermite could certainly react with a little acid or moisture. However, these don't fit the bill as "orange substances."
Flash powder - Often used in special effects and flares. This blend of oxidizers and reactive metals would have to have been intentionally placed on the rocks, since they wouldn't have remained stable for long, out in the open air.
Reactive Groups - Many organic groups react with violent decomposition or exotherms when sparked, touched, or heated. Although certainly far-fetched, someone fooling around with picrates, nitrates, or perchlorates could have inadvertently doped the stones in question. There's at least a few dozen of these salts around, and many (iron perchlorate, mercury fulminate, ammonium picrate) may actually be orange or yellow-colored.