Deep inside a cave on Dragonstone Island, the King in the North, Jon Snow, found a rich mine of shiny black dragonglass in episode four of Game of Thrones’ seventh season. This is an encouraging find for the inhabitants of the Realm because Dragonstone is the only material (other than extremely rare Valyrian steel) that can kill the murderous White Walkers, who are fast approaching.

But dragonglass isn’t just found in Game of Thrones: It’s also scattered around Earth in the form of a volcanic rock called obsidian. It’s too bad Snow doesn’t have access to the mountains of Earth, because they’re relatively rich in the sharp stone.

A finely formed piece of dragonglass in 'Game of Thrones.'

Still, Earthly dragonglass can’t be found just anywhere. Obsidian is formed, in particular, by explosive volcanoes, not the tranquil types found in Hawaii. Its essential ingredient is the thick, gooey molten rock, or magma, that lies below these mercurial mountains. This particular type of magma has a high concentration of silica (over 70 percent), which makes it more viscous — and thus more explosive. As it traps heat and gases, it creates immense build-ups of pressure. Eventually the top pops off, and the magma violently bursts out.

Obsidian, however, doesn’t form as viscous magma explodes out of these volcanoes. Rather, it forms as magma oozes out of different cracks or vents in the mountains, which allows it to cool in a layered, glassy form that leads to obsidian. As magma escapes from the ground or gets close to the surface, water inside the molten rock rapidly steams off, making the already thick substance even thicker. Free of water, it quickly cools in place.

The rapid speed at which magma cools is responsible for obsidian’s glassy purity. It hardens long before anything else can crystallize inside, leaving it unblemished. This same magma, if cooled for eons underground, results in a totally different type of rock: granite. This common type of rock (look at any wall in Yosemite) is filled with different minerals, like grey quartz and pinkish feldspar.

A fat slab of earthly obsidian.

So, if one needs to hastily find obsidian on Earth — perhaps to thwart an army of wicked, partially decayed humanoids — one should visit places where explosive volcanoes exist or have existed. In the United States, that means the West — places like Oregon, Northern California, Washington, and Idaho. Yellowstone National Park is home to an impressive amount of dragonglass, but even the King of the North would be prohibited from taking it from these sacred (that is, government-protected) park lands.

Back in the Realm, Snow is now feverishly mining dragonglass from a cave. Fortunately, the glass stone naturally breaks apart into large, knife-like shards, perfect for lodging into the throat of a menacing White Walker.

Weakenss: Dragonglass.
Photos via HBO (1, 2), Kevin Walsh, HBO / Game of Thrones Wiki