What makes a gemstone?

Generally speaking, a gemstone is a stone that is beautiful, rare, and durable (resistant to abrasion, fracturing and chemical reactions). Some minerals can be very beautiful, but they may be too soft and will scratch easily (such as the mineral fluorite). Fluorite is extremely colorful and pretty but has a hardness of only 4 on the Moh's hardness scale and has four perfect cleavage directions, which makes it only an oddity as a cut gem. Others are too common and are given a semi-precious status (such as agate). Most gemstones have good hardness (above 5) and a high index of refraction (the higher the index of refraction the greater the sparkle). All gemstones have some characteristics falling short of perfection though; even the seemingly perfect Diamond has four directions of cleavage. Note that the most common mineral in the Earth's crust is quartz, with a hardness of 7. Since quartz is everywhere (especially in common dirt and dust), any material that is softer may be scratched during ordinary wear.

A few gemstones are mineraloids (not true minerals) and are included below: opal, amber, and moldavite. Pearls don't even qualify as mineraloids, as they are not only the result of an organic process plus they are properly composites, containing both the mineral aragonite and the protein conchiolin.

While almost any mineral can be cut in the manner of a gemstone, below is a list of some of the gem kingdom's more prized and recognized members.

What makes a Precious Metal?

Like gemstones, one of the characteristics of a precious metal is its rarity. It could not be "precious" if it were common! Two other characteristics are also important. Foremost is durability - it must not easily corrode away, nor can it be brittle. And that is related to the third characteristic, ductility. This means that the metal must be malleable, that it can be bent, hammered, or otherwise shaped. Gold is the most malleable of metals (it can be hammered into incredibly thin foils or drawn into extremely fine wires), it does not corrode or dissolve except under the most extreme conditions.  It is so durable that nearly all of the gold ever mined is still in circulation or storage.