There are many affordable quartz gems used in jewelry: amethyst, citrine, ametrine, rose quartz, onyx, agates, chrysoprase, rutilated quartz...

There are millions of minerals that exist on the earth, but few of them are prized as gems and used for adornment. Many of them aren't in colors that are usually highly prized for jewelry. Or they're not able to be cut and polished to be made wearable as jewelry.

Often it's a matter of changing taste and times. In Imperial Russia and in the Victorian era, the mineral malachite and other opaque minerals were highly valued as gemstones jewelry. It is often a by-product of copper mining, and is distinguished by a brilliant green color, with dark concentric circles of color swirling through it. It's not as highly valued because it's easily available and not as expensive as an emerald or ruby. But there is an entire room devoted to malachite in the Russian museum, the Hermitage, as testament to its desirability among the most privileged class of people.

There have been discoveries that indicate that malachite was mined in Egypt as early as 4,000 B.C. It's a soft gem, and easily carved and shaped. It polishes to a beautiful, rich sheen. But still is not highly prized in the U.S. as a gemstone.

Very often some minerals simply shouldn't be used as gemstones, but because of their beauty, people wear them as such anyway. A moonstone is one such gem. It's relatively soft, with a rating of 6 on Moh's scale of mineral hardness, compared to a diamond's hardness rating of 10. It's a type of mineral called orthoclase, but when it exhibits a translucent, milky quality, it's then called moonstone.

Other minerals exist, but are simply not prized for gemstones. Among the more common minerals, quartz stands out as one that can be used for jewelry, as well as decorative items.