St. John's Island, Egypt

Zabargad Island (Egyptian Arabic: جزيرة الزبرجد‎‎ Geziret El Zabargad, also known as St. John's Island in English) is the largest of a group of islands in Foul Bay, Egypt. It covers an area of 4.50 square kilometres (1.74 square miles). It is not a quaternary volcanic island, but rather is believed to be an upthrusted part of upper mantle material. The nearest island is known as "Rocky Island". The island is slightly north of the Tropic of Cancer, and its highest point is 235 metres (771 feet).

"Before the mid 270s [BCE] Ptolemaic activity along the African coast of the Red Sea had been limited with few significant results other than the discovery of the 'topaz' deposits on the island of Gazirat Zabarjad southeast of Ras Banas.... 2. Pliny, HN 37, 108 refers to a large topaz that was brought back to Egypt as a gift for Ptolemy I's queen Berenice I."

The island is considered geologically unique as it is uplifted mantle, a fragment of the sub-Red Sea lithosphere. Rocks on the island are mainly lower crustal metamorphic rocks. The island became present above sea level after African and Asiatic continental plates converged to cause rocks in the lower crust to be uplifted. The island contains three masses of peridotite, which are rich in the gemstone peridot (olivine). The island is believed to be the first discovered source of peridot, which was called topazios in ancient times, hence the Greek name for the island, Topazios. Layers of spinel-lherzolites with anhydrous Al-diopside pyroxenites and hydrous Cr-diopside pyroxenites can be found too on the island.

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