40 Year Professional Gemmologist reports:
Black diamond is in most respects the same as other diamonds only it contains so much included material (mostly graphite) that it appears black. It has to be cut using exactly the same methods as other diamonds. There has been a fashion fad in the last few years for black diamonds but there are many other black stones which work just as well. In the past black diamond crystals were just thrown in with all the other industrial stones that were mined. It has little value and the only serious cost is involved with the cutting. A price of around $60 per carat is about fair. Do not think of it in terms of an investment as its only value is as a fashion fad which will not last and when the fad is over you will never find anyone to to buy the stones off you.