hardness

Precious Stones Explained!
The mineral to which the term " precious stone" is applied, must be adaptable for jewelry or ornamental purposes and must possess beauty, hardness, and rarity.
The beauty of a precious stone or gem consists of its color or colorlessness, brilliancy or softness of luster, and transparency. To take a high and lasting polish, a mineral must be hard, and many stones that would otherwise be highly valued are low in the estimate of worth because they do not possess of sufficient hardness to make them endure the wear and friction to which a precious stone is subjected when used in the form of jewelry. The rareness of precious stones has a decided effect in determining their values. For instance,1984 Ugg Fox Fur Tall Black Boots, the crocidolite, commercially known as tiger-eye, was sold by the carat some years ago, and was largely used in the making of fine jewelry. Today, this material is so plentiful that it is no longer classed among the higher gems, but serves for cameos and intaglios like chalcedony and onyx.
The changes of fashion have much to do with determining the market value of precious stones. Amethysts, topazes, cat's-eyes, aquamarines, alexandrites, and even emeralds and opals have been eagerly sought for at times and then again neglected for other gems, causing a sensible difference in the value of these stones.
It has been a mooted question as to the proper dividing line between stones that deserve the title precious, and those, which should be placed in a so-called semi-precious or lower category. To draw such a line is hardly possible, as neither hardness, rareness, nor value would be a positive test--some of the hard stones, like zircon and almandines being less valuable than softer opal, while the diamond, one of the most plentiful of precious stones, is at the same time, one of the most valuable.
Neither can price be taken as a complete test, because fashion makes a turquoise, an opal, or an emerald much more valuable at one time than at another, All precious minerals used for ornamental purposes, from the diamond to quartz, or chalcedony, may properly be termed precious stones.
The Final Word On Precious Stones
The most precious stones are the diamonds, emeralds, rubies and sapphires. The pearl is oftentimes classed with precious stones. Although strictly speaking while it is not a stone it holds an esteemed place in jewelry.