Blue Diamonds
We're used to thinking of diamonds as clear stones, however they are found naturally in a variety of different colours. These colours are produced by various trace elements that are in the stone along with the basic carbon and the nature of the element determines the colour of the diamond.
Of all the natural varieties blue diamonds - usually caused by traces of boron - are amongst the rarest and most precious. The actual colour of natural "blue" diamonds varies considerably, often they have a very light, subtle shading more akin to steel-blue, sometimes the colour is deep and vibrant.
Value
It's said that most jewelry dealers today have probably never been in a position to sell a natural blue diamond, so it seems unlikely that the pirates of old would have had one of them in their booty. If they had then they might not have believed what it was worth - they could probably have retired on the proceeds of its sale. In 2007 Sotheby's of Hong Kong sold a 6.04 carat blue diamond at auction for just under US $8 million.Given the rarity and value of blue diamonds it's not surprising that they have often been associated with royalty.
Of course there are "blue diamonds" that are within the means of the rest of us to buy. However these are usually synthetic or have been artificially coloured by methods such as radiation or heat treatment. There's nothing wrong in principle with such stones, if you buy quality gems then they can look beautiful in rings, necklaces, earrings, etc. It's just that they're not the true treasure of which we dream.
Famous
In terms of fabled treasures the cursed Hope Diamond is probably the most famous blue diamond in the world. Legend states that this large gem was stolen from the eyes of a Hindu idol and since then has brought misfortune to all who possess it.Other famous examples with a less worrying legacy include the Blue Heart (sometimes called the Eugenie Blue) and the De Beers Millennium diamonds which include the Millenium Star and the Heart of Eternity.