DIAMONDS are formed in more ways than one. While it was believed earlier that the beautiful crystals were retrieved from deep mines of igneous or metamorphic rocks of the mantle formed under conditions of high temperature and pressure, new research reveals that there are other ways by which they are formed in nature.

Meteorite impaction on the earth's surface is one instance which has led to the formation of these precious stones. Work conducted by R M Hough and others from the department of earth sciences at the Open University in Milton Keynes, UK, now adds another theory regarding the origin of these surface diamonds. It is believed that diamonds were present in the parent meteorite and were expelled onto the earth's surface as a result of the impact. Earlier studies have shown the presence of these crystals in leaded meteorites and urelites. It was also assumed that diamonds in a meteorite originate due to the high gravitational pressure exerted on it during its fall and may have interstellar origins (Nature, Vol 378, No 6552).

Recently, scientists discovered tiny diamonds, identified from an elusive noble-gas bearing component in chondritic (spheroidal mineral grain bearing) meteorites. The rare gas component gave them away as being interstellar in origin, formed due to vapour condensation in stellar atmospheres.

Hough et al conducted their study on material collected from the 23 km diameter Reis crater in Germany, where they found small quantities of cubic and hexagonal diamonds alongwith cubic diamonds interspersed with silicon carbide. The discovery of such diamond composites is the first of its kind, formed by undergoing condensation in the hot vapour cloud of the impact.

Impact diamonds have also been found in clay, marking the boundary between the Cretaceous and Tertiary periods when the earth was bombarded by a large number of meteorites. Carbon and nitrogen isotopic data revealed these diamonds to have been formed due to impact or from within the fireball. The chemical structure and isotopic markings of these impact associated polycrystalline diamonds is distinct. They preserve the macroscopic crystal layout of the graphite crystals from which they originate due to shock metamorphism.

Another rare and unusual form of polycrystalline black diamonds have been found in the sedimentary formations in Bahia, Brazil and in Ubangi, Central African Republic. These formations are called carbonados, which are extremely hard polycrystalline aggregates. Enclosed within their porous matrix of randomly arranged crystals are octahedral and cubic diamonds. The mechanism of formation of these diamonds is not yet clear, though suggestions range from their origin as carbon subduction in the mantle, to shock metamorphism during impact of irradiation of organic matter.