Despite
DMan's comment about jade not being particularly valued in Western society, jade (either of the two kinds,
jadeite and
nephrite) is valuable to
gemstone aficionados.
Jade jewelry pieces are measured in millimeters across, rather than carats like most gems. (Since jade for jewelry is usually cut as flat or cabochon pieces, or round beads, this is more practical than it would be with gems that are faceted.) The color and the presence or absence of inclusions of some other mineral are the major forces in determining the price of a piece of jade. Sometimes jade is dyed to appear a more valuable color than it originally was, but this can be identified by a skilled examiner. The color can also be altered by too much heat, most commonly from machine polishing. (It can also turn grayish if left in direct sunlight for too long.)
Some other green stones have been mistakenly (or purposely, to make them sound more valuable) called "jade." For example "Korea jade" is actually serpentine; "Transvaal jade" is a grossular garnet. Other materials that have been called or passed off as jade are green feldspar, quartz, aventurine, chrysoprase, jasper, soapstone, malachite, marble, and even glass.