Most gemstones come from the earth’s crust. Peridot does not.

Peridot is composed primarily of magnesium iron silicate, formed in the upper mantle, tens of kilometres below the surface. It is stable there, but when mantle material is carried upward through volcanic activity, peridot comes with it. Basalt frequently contains peridot crystals, because basalt itself is the product of cooled mantle lava. Some beaches in Hawaii are green because the sand contains large quantities of peridot particles, weathered from nearby volcanic rock.

Peridot also occurs in meteorites. The Pallasite meteorite found in Siberia in 1749 — known as the Pallas Iron — contained peridot crystals embedded throughout its structure. This type of meteorite was subsequently named pallasites. It indicates that peridot did not originate on earth alone: it existed in the early solar system, before the planets formed, as one of the fundamental minerals from which rocky bodies are built.

Zabargad is a small island in the Red Sea, approximately fifty kilometres from the Egyptian coast, less than seven square kilometres in area.

There is no fresh water on the island. There are no permanent residents. There is peridot. Ancient Egyptians were mining it here before 1500 BCE. Pharaonic texts refer to the island as the Serpent Isle — reportedly because it was once heavily populated with snakes, which early miners had to clear before they could work. Later, Ptolemaic documents began using the name Zabargad, from the ancient Arabic word for peridot. The island’s name and the stone’s name became the same word.

The veins on the island run through ultramafic rock — peridot crystals embedded in the fissures of serpentinite and pyroxenite. The finest material is a pure yellow-green, highly transparent, with crystals sometimes reaching several centimetres. The ancient extraction method was to heat the rock with fire during the day and douse it with cold water at night, using thermal expansion and contraction to crack the stone, then lever out the crystals with tools. The method was extremely inefficient. It continued for several thousand years.

The Zabargad mines were largely exhausted by the mid-twentieth century. There is no active mining on the island now. Geologists and archaeologists visit occasionally.

Ancient Egyptians called peridot the gem of the sun. It emits a yellow-green light in sunlight and is nearly invisible at night — ancient texts record that miners searched for peridot veins by lamplight in the dark, because the stone does not glow in darkness; its location only becomes clear in daylight.

Cleopatra was known for her emerald collection. Ancient sources contain extensive records of her emerald jewellery. But emerald and peridot overlap in colour — emerald is pure green, peridot is yellow-green, and under the lighting conditions of antiquity the two were sometimes difficult to distinguish. Gemologists who re-examined ancient texts and surviving artefacts in the twentieth century proposed a revision: a significant portion of what the ancient sources recorded as Cleopatra’s emeralds may in fact have been peridot from Zabargad.

This revision has no definitive resolution. But the Zabargad mines were still operating during her reign, and the primary sources of emerald had not yet been discovered at scale. Both facts give the hypothesis some ground to stand on.