You will see lots of sellers listed "lab" diamonds - usually of very large size and fancy cuts.  They are often listed with a grading that is usually associated with real stones.

Well, I an tell you, from my personal and painful experience that these "lab diamonds" are nothing of the sort.  Usually to have claimed to have been made in Russia,  I bought a couple of these stones and discovered within minutes that these are not diamonds.  I tested my "diamond" with a piece of ruby, and, as anyone who knows anything about diamonds or mineralogy, the ruby should not have marked the "diamond".

Needless to say, my ruby scratched the "diamond" like a knife through butter ie it was not diamond - who knows what it was really made of - obviously the seller (Bijouxgems) refused to tell me.  I returned the goods, they denied reciept, ignored my proof of posting and refused a refund.

Synthetic diamonds are certainly available - they are usually small (around 1ct), often yellowish and not cheap if it is a good synthetic, although cheaper than a mined diamond.

I speak as an experienced mineral and meteorite collector and I am trying in my little way to ensure that eBay listings are as honest as possible and that the rogue traders which unfortunately are rife and unscrupulous are pushed off eBay.

Anyone offering "lab diamonds" starting at very low bids will almost certainly be fakes - know your buyer!!!