FLINT

A sedimentary rock which may be blue, green, grey or black when it is freshly extracted. It develops a white crust when it is exposed to the atmosphere.

Most flint is a sedimentary form of chalcedony. It is composed of silica, which is the main constituent of most quartz, chalcedony and opal. It may may also contain calcite, gypsum, hematite (small traces) and certain fossils e.g. amoeba. It's composition depends on the mineralogical make-up of the surrounding rock.

It is fine grained and despite it's hardness is easy to shape and was one of the first materials used by stone age people for arrowheads. Later it was used to ignite gun powder which fired bullets because it creates a spark when struck. Today it is important in the manufacture of glass and pottery and provides material for road construction. It is also used as an abrasive. Flint can be used to make semi-precious jewellery and small pieces may be polished and used for ornaments.

It's name is derived from the Swedish word flinta which means splinter of stone.

Flint originates from the sea floor and forms when silica gel in sedimentary limestones hardens (organically) or when limestone is replaced by silica (inorganically).

It is found in chalk zones of Western Europe and in India and the Pacific Ocean, Kent, Sussex (UK), the Rhine Valley (Germany), California and Oregan (US) and North Wales.

GABBRO

An internal igneous rock which forms deep beneath the earth's surface and is therefore plutonic. It is coarse grained and is speckled grey or black in colour.

The best known varieties are hornblende gabbro and quartz gabbro. To be classified gabbro minerals must contain greater than 50% anorthite. Other varieties are Norite and Trocolite, which is also known as olivene gabbro (after it's accessory mineral). Norite contains greater than average concentrations of plagioclase feldspar which is an essential component. Layered gabbro forms in alternating dark and light bands.

Gabbro contains pyroxenes and may also contain hornblende, olivene, quartz and orphopyroxenes. It has a more variable chemical composition than basalt.

Most gabbro is too fragile to use in building construction but some small pieces may be polished into decorative slabs. It is an industrial source of chromium, nickel, platinum and magnetite.

Most gabbro forms in dykes, sills and stocks which may be several kilometres in diameter. More rarely it may be found in large sheets.

It may be found in Ontario (Canada), Harz Mountains (Germany), Isle of Skye (Scotland), California and Montana (US).