1.  Metallic minerals.  These include things most of us would think of if we were asked to name some minerals.  Familiar examples are copper, silver, mercury, iron, nickel and cobalt.  Most of them are found in combination with other things—­as ores.  We get lead from galena, or lead sulfide.  Tin comes from the ore cassiterite; zinc from sphalerite and zincblende, or blackjack.  Chromium that makes the family car flashy comes from chromite.  Many minerals yield aluminum.  Uranium occurs in about 50 minerals, nearly all rare.  Twenty-four carat gold is a metallic mineral.  A 14 carat gold ring contains 14/24 or 58% gold.

An average sample of earth contains 9% aluminum, 5.5% iron, .01% zinc, .008% copper, .004% tin, .002% lead, .0005% uranium, and .0000006% gold or platinum.  It would be hopelessly expensive to recover such metals from an average ton of earth.  That’s why metallic minerals are taken from concentrated deposits in mines.

Many valuable minerals are found in veins running through rock.  Veins can be formed when:  (a) mineral-laden ground water seeps into cracks, evaporates, and leaves mineral grains that build up into a vein; (b) hot water from deep within the earth fills cracks, then cools and deposits much of the material in solution as minerals in a vein—­sometimes including metals such as gold and silver; (c) molten gaseous material squeezes into cracks near the earth’s surface, then slowly hardens into a vein.

2.  Nonmetallic minerals.  These are of great importance to certain industries.  You will find them in insulation and filters.  They are used extensively in the ceramic and chemical industries.  They include sulfur, graphite (the “lead” in pencils), gypsum, halite (rock salt), borax, talc, asbestos and quartz.  Undoubtedly, you’ll have some nonmetallic minerals in your collection.  Rocks containing asbestos are especially handsome and varied.

3.  Rock-forming minerals.  These are the building materials of the earth.  They make mountains and valleys.  They furnish the ingredients of soil and the salt of the sea.  They are largely silicates—­that is, they contain silicon and oxygen. (Silicon is a nonmetallic element, always found in combination with something else.  It is second only to oxygen as the chief elementary constituent of the earth’s crust.)

Other rock-forming minerals are the large family of micas, with names like muscovite and phlogopite.  There are the feldspars, including albite and orthoclase.  Others are amphiboles, pyroxenes, zeolites, garnets and many others you may never find or hear about unless you become a true mineralogist.

A rock may be made almost entirely of one mineral or of more than one mineral.  Rocks containing different combinations of the same minerals are different.  Even two things made of the same single mineral can be quite different.  Carbon may turn up as a lump of coal or a diamond.