In the remote desert town of Coober Pedy, in South Australia, summer temperatures can reach 127 degrees—so hot that many residents live below ground, where the temperature is a constant 75 degrees.

This tradition began 100 years ago when the first opals were discovered, and so was born an underground cottage mining industry that endures today.

Only 5 percent of opals found worldwide are designated as precious, with a fiery play of color within them, and 95 percent of those come from Australia.

Unlike other gemstones, opals are not minerals but instead made of microscopic spheres of quartz—silicon dioxide—that form very slowly in sediment layers at low temperature in the presence of water.

Perhaps it’s not surprising then, that opals are 10 to 20 percent water.

As they form, they may replace minerals in existing fossils. Some of the Coober Pedy opals inherit the fossil forms of ancient sea creatures.

Here, the Australian government has discouraged large-scale mining by limiting prospectors to single claims.

The result is more than 250,000 small mine shafts in the area, some of which have been converted into underground homes, hotels and businesses.

To excavate a new home costs about the same as building one above ground. But the diggers may uncover more gemstones in the process, which helps to subsidize the cost of the home.

Now that’s a valuable opal.