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complex numbers and complex variables
Science; General science
A natural and extremely useful extension of the familiar real numbers. They can be introduced formally as follows. Consider the two-dimensional real vector space consisting of all ...
cave
Science; General science
A natural cavity located underground or in the side of a hill or cliff, generally of a size to admit a human. Caves occur in all types of rocks and topographic situations. They ...
coir
Science; General science
A natural fiber, also known as coco fiber, obtained from the husks of the coconut ( Cocos nucifera ). Although coconuts are grown for copra throughout the tropics and into the ...
volcanic glass
Science; General science
A natural glass formed by rapid cooling of magma. Magmas typically comprise crystals and bubbles of gas within a silicate liquid. On slow cooling, the liquid portion of the magma ...
pitchstone
Science; General science
A natural glass with dull or pitchy luster and generally brown, green, or gray color. It is extremely rich in microscopic, embryonic crystal growths (crystallites) which may cause ...
opal
Science; General science
A natural hydrated form of silica. Opal is a relatively common mineral in its nongem form, which is known as common opal and lacks the play of color for which gem, or precious, ...
emery
Science; General science
A natural mixture of corundum with magnetite or with hematite and spinel. Emery has been used for centuries as an abrasive or polishing material. Because the mixture is very ...
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