We have found a nice article on the web from "Wildgoat2005" a reviewer for eBay. He have tested more than 69 jade items. Beneath we have mad e a link so you can read this informative article.

Conclusion from the review by "Wildgoat2005":

" I have tested 69 jade items.  Three are know jadeite items from two eBay dealers and I obtained a density from 3.200 to 3.300.  All there had no problems in passing the hardness test.  I tested two pieces of known nephrite rocks from an ebay dealer and obtained density of 2.833 and 3.000.  I obtained five pieces of nephrite with different colors from a Texas rock company directly and obtained density: 2.929 to 3.083 for 3 green nephrite rocks; 2.725 for a black nephrite and 2.960 for a pink nephrite.  I tested over fifty jade items from three eBay dealers: 39 jade items have density below 2.980 and 15 items are above 2.980.  The lowest is 2.529 and the highest is 3.121  Many Hongshan pieces actually have densities around 2.90 -3.00 but their color and texture just do not look like typical nephrite.  Surprisingly the darker jade items and the one with white patina are actually made of nephrite (they also passed the hardness test).  I only tested one zong and it failed the density test.  Therefore, I believe any jade item that have a density of at least 3.000 and pass the hardness test, then it is very likely that they may be made of nephrite.  The one that have density around 2.900 - 3.000 will be inconclusive and require some more studies.  Also please bear in mind that many authentic and ancient Chinese jade items were actually not made of either jadeite or nephrite. "