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187 Topic Pages Found in Geology
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Open Agates
Agates

( ăg'ĭt ), translucent, cryptocrystalline variety of quartz and a subvariety of chalcedony . Agates are identical in chemical structure to jasper,

Open Alexandrite
Alexandrite

Alexandrite, a rare variety of chrysoberyl, is prized for its remarkable optical properties. In daylight, which is rich in shorter wavelengths, it

Open Alluvial fans
Alluvial fans

A fan-shaped, low-angle depositional landform of predominantly fluvial origin, characterised by a distributary pattern of channels, and composed of

Open Amethysts
Amethysts

Boasting the famed colour of royalty, amethyst sets the colour standard for all other purple gemstones. A variety of QUARTZ , amethyst has long been

Open Andesite
Andesite

THE VOLCANIC EQUIVALENT of diorite , andesite is fine grained or porphyritic, and primarily consists of the plagioclase feldspar minerals

Open Anorthosite
Anorthosite

AN INTRUSIVE IGNEOUS ROCK, anorthosite is composed of at least 90 per cent calcium-rich plagioclase feldspar – principally labradorite and

Open Appalachian Region
Appalachian Region

Region of eastern USA, corresponding roughly to the area covered by the Appalachian Mountains. Stretching from the Gulf of St Lawrence to central

Open Aquamarines
Aquamarines

The clear blue of aquamarine attracts jewellery makers and gem collectors alike. Its tone flatters all skin colours and harmonizes with all precious

Open Archaeology
Archaeology

( ärkēŏl'Әjē ) [Gr.,=study of beginnings], a branch of anthropology that seeks to document and explain continuity and change and similarities and

Open Arkose
Arkose

A RELATIVELY COARSE SANDSTONE, arkose consists primarily of quartz and feldspar grains with small amounts of mica. It has a high feldspar content