It takes a lot of hands on experience to be able to grade the quality of jade. Put imitations, simulants, synthetics, assembled and treated material into the equation and it becomes much more difficult.
It would take a thick book chockful of pictures to explain, but I can't do that so I'll be brief here! Here are some guidelines as to what makes good quality jade:
Greens and Lavenders command most, followed by fancy colors like red, yellow, black etc. Look for pure and intense color. Undetones of grey or brown are not desirable.
Color alone is not enough. Good jade must also have a good translucency and texture. Visualise a glowing glob of pure color that has a glassy or honey-like texture! It should also be free from blemishes such as clouds (white inclusions), black spots, distracting veining and host rock material.
Cut & Carat
The ideal size for a cabachon is approximately the dimensions of a man's thumbnail. It should be balanced, sufficiently thick and polished to a glossy luster. The luster should look as if it is wet or oily. You may have observed many stones of rather attractive coloring that unfortunately look dull and/or dry. This is undesirable. It is not known widely, but it takes a goodcraftsman is able to "read" the graining of the rough material to get the best polish. Jade that is cut too thin projects a false translucency and suffers durability problems.
A premium is paid for antique jades, antique jade jewelry, and good jade carvings.
Look out for jades that have been bleached, dyed or both, assembled, coated etc. Also be aware that jade has many simulants e.g. Chrysoprase, Aventurine Quartz, Maw-sit-sit, Serpentine, Bowenite etc....
Hope that helps!
I LOVE Gems and collect Jade as a hobby!