How to Identify the Natural Stones Onyx and Moonstone

Moonstones and onyx are two popular stones often found in jewelry designs. Although both stones can be black in color, there's little physical resemblance between the two. While onyx stones are always opaque, moonstones never are -- they're translucent or semitransparent. Moonstones and onyx find parallels in jewelry design, however. Both stones are used in cameos and both can be found as accents to rings, necklaces, earrings and other jewelry.

The word "onyx" is Greek for fingernail or claw. Legend has it that Cupid clipped Venus's nails as she slept beside the Indus River. The clipped nails fell in the water and the Gods turned them into stones as they settled in the riverbed. These stones became known as onyx in honor of their original state as Venus's fingernail. What is commonly considered black onyx is agate -- a banded form of the mineral chalcedony. Agate bands appear in contrasting colors, with no agate stone ever existing as one solid color. To create solid black onyx, banded black agate is treated with sugar and hot sulfuric acid, which turns the stone completely black.

In its natural form, banded black onyx has a black base with white parallel bands across the top. Sardonyx has a brown base with white bands, and cornelian onyx has a red base with white bands. In niccolo onyx, the white bands are thin -- almost translucent -- making the black base underneath visible and giving the appearance of dark blue or gray bands instead of white. Onyx stones can be identified by this pattern -- a base color with straight, parallel, white bands. Other agates exist with wavy or concentric white bands, but these types are never classified as onyx.

Hindu mythology describes moonstones as moonbeams solidified. Romans considered them frozen moonlight. Both cultures -- and many more -- have tied the shimmering stone to rituals associated with varying moon phases. From its ancient beginnings as a talisman of luck and good fortune, the Art Nouveau period of the early 20th century brought the stone its first commercial notoriety. Jewelry designers of this era incorporated moonstone into everything -- from men's tie clips, watch chains and cufflinks to women's necklaces, earrings and bracelets.

Moonstones have a distinctive, unmistakable quality: They appear to glow. This rare trait gives moonstone an opalescent color play, which creates a different appearance at different angles. This trait is also the single best way to immediately recognize a moonstone from all other stones. Often cut into cabochons -- or polished domed shapes -- moonstones comes in a spectrum of hues, including gray, brown, yellow, green and pink. Moonstones are often translucent or semitransparent. They are never opaque. The moonstone known as "cat's eye" reflects a narrow band of white light in the form of a four-spoked star.