Popping Rocks

Popping Rocks are strange things, really. Toss one about the size of a walnut into a campfire and within two or three minutes, there will come a load "POP!" and the bloody thing will expand to the size of your head. Once they've popped, they are very light, very porous and quite easy to crumble in your hands.

For the first 20 years they were known about, Popping Rocks were used merely as an entertainment for children. Lord knows there were plenty of them about, since they are a byproduct of mining for Blue Silver, which we use to build His Majesty's Aerial Armada. But some years ago, a scientist found that crushed popped Popping Rocks could be blended with other chemicals to create a cheap and very efficient fuel that burned much hotter than coal. Suddenly, Popping Rocks became quite valuable.

Popping Rocks are found in various locations throughout the Empire, Europe and North America. Strangely, neither Blue Silver nor Popping Rocks have ever been found in the southern hemisphere.