Man's physical universe

xanabras

188 THE EARTH AS MAN'S ABODE

The majority of precious stones are only special varieties of common

substances. Aluminum oxide (corundum) is a common mineral;

but when it occurs in blue or red colors, we have sapphires and rubies

worth from $500 to $5000 a carat.

Most of the gems owe their colors to impurities. Thus ordinary

quartz (silicon dioxide) is not highly valued, but quartz colored with

a little iron or manganese oxide gives us our highly valued purple

amethysts. Quartz occurs as rock crystals, rose quartz, and milky

quartz. A dense, translucent variety of silicon dioxide is known as

chalcedony, some forms of which are agate, sardonyx, carnelian, jasper,

onyx, and opal. Of the more than 1500 known minerals, approximately

50 are used as gems.

The diamond is a crystalline form of pure carbon which was probably

produced from carbon trapped in molten rocks.

Most of the precious stones may be synthesized today. With the

discovery of this synthesis, superstition surrounding precious stones has

decreased, and their values have changed. Now they are valued for

what they are rather than for their rarity.

Minerals Are Identified by Their Physical and Chemical Properties.

It is clear that one may have to examine a great many properties

before a sure identification can be made. This is why a knowledge of

chemistry is always a useful and sometimes a necessary supplement

to microscopic examination and physical tests.

Quartz resembles calcite in appearance, but calcite can be scratched

with a knife and quartz is too hard to be scratched with a knife. The

hardness of a mineral is of considerable aid in its identification. The

hardness scale is as follows: 1, talc; 2, gypsum; 3, calcite; 4, fluorite;

5, apatite; 6, orthoclase feldspar; 7, quartz; 8, topaz; 9, corundum;

and 10, diamond. The hardness of a mineral is determined by finding

which of the above minerals will scratch it and which of these minerals

it will scratch.

The specific gravity, i.e., its weight relative to the weight of an equal

volume of water, is another important physical property. Color, luster,

and the manner in which minerals break — either fracturing or forming

cleavage planes — are terms frequently used in describing minerals.

Coal Was Formed from Liixuriant Land-plant Growths.

Today a brownish-black substance known as " peat " is being formed

in bogs where plants grow luxuriantly and partially decay as their

dead remains fall into the water. Thus peat beds forty feet thick have

been formed.