 Lying some 150 km WNW of Abu Dhabi and 90 km from the nearest mainland coast, Qarnein is instantly recognisable at a considerable distance on account of the two protruding hills or 'horns' from which it takes its name. Geologically, as with certain other Abu Dhabi islands, the hills of Qarnein are the surface expression and net result of salt dome diapirism whereby buried  salt deposits, migrating under the weight of the overlying strata, ultimately burst upward to the surface following lines of weakness. On Qarnein these are exemplified by volcanic plug-like extrusions rising over 50m above the surrounding surface.
Just 3 km north-south and barely a kilometre east-west, this island supports internationally important breeding populations of several species of seabird. A single colony of up to 20,000 pairs of lesser crested tern (Sterna bengalensis) here is quite possibly the largest assembly of its kind in the world. Bridled Terns (S. anaethetus) can top 10,000 pairs, while the sole colony of swift terns (S. bergii) in the UAE, with over 1000 pairs annually, is also found here. About 300 pairs of sooty gull (Larus hemprichii) nest, attracted by nesting terns which they harry for the fish being carried back to their young. The absence of ground predators is all important, as must be the abundant fish supplies close inshore.
 In the winter, when the summer nesting terns are away from the island, red-billed tropicbirds (Phaethon aethereus) take up territories on the island rocky 'horns'. Around 100 pairs nest, this being, as with the sooty gull, the Arabian Gulf's single most important station.
Extensive areas of coral reef fringe the island and a small number of hawksbills (Eretmochelys imbricata) come ashore to nest. The island, privately owned by a member of Abu Dhabi's ruling family receives full protection, extending to birds and turtles and their eggs, and to fisheries, while there is, furthermore, no public access.
Surface scatters of pottery from the early first millennium AD to the Late Islamic period provide evidence of early maritime commerce through the Arabian Gulf.
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