Jade refers collectively to two different minerals-jadeite (also called soft jade) and nephrite (also called "ancient jade" or hard jade).  Jadeite and nephrite have different physical and chemical characteristics:  They also have different crystal structures, colors, specific gravities, refractive indices and hardness.

Most jade on the market is composed of nephrite:

Jadeite is more rare and valuable:

It is difficult to distinguish nephrite jade from jadeite jade by only visual inspection.  Specific gravity determination is the most reliable of simple I.D. methods for distinguishing the two.

Here are a few main differences between nephrite and jadeite:

• Hardness: Nephrite: 6 - 6.5; Jadeite: 6.5 – 7. Jadeite is slightly harder and denser, and is said to take a polish better
• Specific Gravity: Nephrite: 2.90 - 3.02    Jadeite: 3.3 - 3.5
• Color: Nephrite ranges mainly from mid to dark green or grey-green, but it can also be white, yellowish or reddish. Jadeite shows more color variations, including yellow, lavender-mauve, pink and emerald-green.
• Grade: Interestingly, jadeite and nephrite are graded in reverse.  The best nephrite is clear and white, also know as "water jade."  However, the best  jadeite today is imperial green.
• Chemical characteristics: Jadeite is a silicate of sodium and aluminium and Nephrite is a silicate of calcium and magnesium.
• Fibrous structure: Nephrite has a looser interwoven filtrons structure, whereas Jadeite has a very tight interlocking granular structure.