I have purchased well over $30,000 worth of gold on eBay. My experience with scrap gold jewelry has been filled with disappointment when my purchases that promised to be filled with diamonds were filled with glass and Cubic Zirconia instead, about 90%  of the auctions for diamond filled scrap jewelry I have purchased on eBay,  has turned out to be faux gems.
 I can only assume that jewelry owners have been selling of the stones before auction, or the gems are being stolen by jewelers, and replaced with replica stones.
 My advice on buying scrap gold or silver jewelry on ebay, is pay according to the weight of the precious metals, forget about the gems, chances are they are not real.
 Diamonds are easy to identify, you need only a black light. Diamonds fluoresce blue, CZ will fluoresce a mustard yellow, and glass does not fluoresce it will look black instead.