A Blue Diamond can range from a brilliant deep aqua through to a bright peacock blue.
A Blue diamond is one of the fancy colored stones. These are the very rarest and most expensive. Blue is one of the rarest and most desirable colors. It has a distinct blue color and some stones will show blue in both daylight and incandescent (artificial) light. These are expensive stones. A more common fluorescent stone will show blue in daylight only.
The Hope Diamond, which resides in the Smithsonian Institute in Washington D.C, is probably the most famous blue diamond in the world, and the largest at 45.52 carats.
Natural blue diamonds are rare enough that most jewelers never get to see one and are unlikely to ever own one.
Mostly, if one sees a blue diamond, it is more likely to be a treated diamond where irradiation and heat treatment has been used to change and stabilize the color. Virtually all blue diamonds you see will have been created this way. Buteven these are rare enough.
Genuine fancy blue diamonds are of course expensive, even more so than the best quality white diamond, carat for carat. You can expect to pay top dollar for a treated blue diamond and as for a genuine natural blue diamond, well a carat could cost about a million dollars.
If you see blue diamonds for sale, in rings or other jewellery or even lose, rest assured it is most likely a treated diamond. Even so, it will be expensive and you should ensure you get a certificate from an independent laboratory to verify the diamond and its characteristics. Such as color, clarity, cut and carat weight.