A look at the breastplate found in King Tut’s tomb you might not realize that it features one of the rarest gemstones in the world.

Tutankhamen’s tomb was filled with statues made of ivory, precious jewelry, and even a complete golden chariot. In one treasure chest, Carter discovered a large breastplate, decorated with gold, silver, various precious jewels, and a strange gemstone. The breastplate shows the god Ra as a winged scarab, made from the yellow, translucent gemstone, carrying the celestial bark with the Sun and the Moon into the sky.

Howard Carter identified the gemstone from the Tutankhamen breastplate as chalcedony, a common variety of the mineral quartz.

Ten years later the British geographer Patrick Clayton was exploring the Libyan Desert along the border of modern Egypt and Libya. Here he discovered some strange pieces of glass in the sand. The pale yellow in color and translucent material seemed to be identical to the gemstone found in Tutankhamen’s tomb.

Two years later he published a short note, suggesting that the pieces of Libyan Desert Glass (LDG) were the quartz-rich deposits of a dry lake.

In 1998, Italian mineralogist Vincenzo de Michele analyzed the optical properties of the gemstone in King Tut’s breastplate and confirmed that it was indeed a piece of LDG.

Libyan Desert Glass (LDG) is almost pure silicon dioxide, like quartz, but its crystal structure is different.  It also contains in traces an unusual combination of elements, like iron, nickel, chromium, cobalt, and iridium.  It is among the rarest minerals on Earth, as it is found only in the Great Sand Sea north of the Gilf Kebir Plateau, one of the most remote and desolate areas in the Libyan Desert.

The origin of desert glass remains uncertain. Glass forms in nature when quartz-rich rocks melt and rapidly cool. Tektites are natural glass formed from terrestrial debris ejected high into Earth’s atmosphere during meteorite impacts. Tektites have been found across Asia, Australia and as far away as Antarctica. If the LDG is a tektite, it formed 28 to 26 million years ago when an impact melted the quartz-rich sand of the desert. The unusual elements found in the LDG are explained as traces of the vaporized meteorite.

However, no impact crater was ever found in the Libyan Desert.

In an alternative scenario proposed in 2013 a comet, composed mostly of ice, may have exploded above the desert. The generated heat burst, an estimated 3,600°F, would be sufficient to melt the upper layers of the sand dunes, forming the desert glass, but without leaving a crater behind.

It’s also uncertain just how the desert glass became part of Tutankhamen’s treasures. Today, caravans rarely cross the Great Sand Sea. Archaeological evidence suggests that an ancient system of caravan routes existed around the Gilf Kebir Plateau, but it doesn’t seem that the routes were used to search or trade for desert glass.  It seems that the piece used for the scarab was discovered by chance or maybe an exotic gift. It remains the only known example where an Egyptian artist used this mysterious material.