The tendency of some minerals to split with relative ease in particular directions along planes is called cleavage. Cleavage is also a factor determining the durability of gemstones. Some gemstones do not exhibit this tendency at all, whereas others cleave in several directions. The number of cleavages is always the same in any one mineral, and the direction of cleavages is constant in relation to the crystal structure of any one mineral or gemstone. It is apparent that of stones having the same hardness, the ones lacking cleavage or having the lesser number of good cleavage directions are the most durable.
Some stones, such as jade and agate, owe their durability to their compact fibrous structure, which makes them very tough and durable even though they are not especially hard.
Several other properties of gemstones, although not always contributing to the beauty or desirability of gemstones, are useful in identifying uncut specimens.
Streak is the color of the mineral when finely powdered or, for softer minerals, the color obtained by rubbing the mineral against a piece of unglazed porcelain or tile. The color of a mineral’s streak is commonly different from the unpowdered specimen.
Fracture is the kind of surface obtained when the mineral is broken in a direction that is not a cleavage direction. Fracture surfaces are described by such terms as conchoidal (like the fracture of glass), subconchoidal, splintery, even, and uneven.
Tenacity is the resistance of a mineral to breakage. Brittle minerals break relatively easily on impact. Malleable minerals, such as gold, may be flattened under a hammer into very thin sheets without breaking. Sectile minerals may be cut with a knife without powdering. Most gemstones, even diamond, are brittle.
It is only natural to value most those gemstones that are not common or easy to obtain. Emerald owes its longstanding popularity to its fine green color, but tourmaline is sometimes found in colors that very closely approach that of emerald and yet sells for considerably less because it is so much more common.
Rarity is not the only factor affecting the value of gemstones. Freedom from internal imperfections, quality of cutting, color, and size must also be considered in cut and polished gemstones. Internal imperfections, such as inclusions and cracks, detract from the appearance of gemstones and interfere with the passage of light between the facets; consequently, gemstones containing these imperfections are not valued as highly as those without them. Poor cutting or polishing detract from the beauty and thus from the value of gemstones. Unpopular or poor color commonly causes gemstones to be less valuable. Rich green emeralds are exceedingly prized, whereas very pale green emeralds are relatively inexpensive. Diamonds that have the least hint of yellow are never valued as highly as pure colorless, pink, or blue stones. Few persons find the yellowish color attractive, unless it is a vivid canary yellow.
Size is important in determining the value of gemstones but not as important as perfection. A badly flawed gemstone of large size may be worth only a slight fraction of the value of a smaller perfect one. Gemstone size is usually measured in carats, a unit of weight, although millimeter size is sometimes used. Five carats is equal to 1 gram and approximately 28⅓ grams is equal to 1 ounce avoirdupois. One one-hundredth (0.01) of a carat is called a point, and this term is often used, especially pertaining to very small gemstones.
The term used to compare the relative weights of minerals and gemstones is specific gravity, which is expressed numerically in relation to water. Water is assigned the value of 1.00. Therefore, at a given temperature a gemstone having a specific gravity of 2.00 is twice as heavy as an equal volume of water. A 1-carat sapphire (specific gravity about 4.00) will be smaller than a 1-carat amethyst (specific gravity about 2.65) because the heavier material will occupy less volume to have the same weight.