Hey Jadies,
Someone earlier had commented that to do a Specific Gravity, one needed expensive or complicated equipment-- not so. All you need is a scale that measures in grams, a cup, a string and something to write with! I use a digital food scale that I got at Walmart ( I use it for everything from weighing little kittens to measuring SG). Because jadeite and nephrite are very dense, the SG of imitators is nearly always much lower. This makes it easier to rule something out (or in) as jadeite/nephrite.
Here are some easy instructions: http://www.johnbetts-fineminerals.com/jhbnyc/articles/specific_gravity.htm.
I use a piece of string to hold the item, instead of the wire that he uses. With a UV black light, a loupe, something like a metal rod to check the chime (bangles only, while suspended from a string), a little digital scale, a bright focused light (phone flashlight works great) you've got your own jadeite/nephrite home testing kit. If you have an expensive or valuable item, or want definite authentication I suggest sending it to GIA (or your country's version), but these screening tests you can do yourself. Research, and read through the saved threads to "train your eyes" to learn what the differences are between Grade A, treated jade, and other minerals a seller may call jade, but are not.
Thanks for the article, Luna. I could never figure out how to measure the specific gravity, so this is a good tip.