Penny...I know there are diamonds near meteor crater Arizona...as I said in my post, it is probably the high pressures of impact that form them...they don't already have to be sitting there on the ground. I suppose there would have to be some carbon in the impact zone. As for the 12,900 BP impact, the article discusses some other geochemical evidence that there was an impact. Not sure about Iridium. If a comet that broke up before impact, might be rather different than, say, the K-T boundary with a 10 km asteroid....
Chris