Blue diamonds are formed by the concentration of boron in the bonding of atoms. As the amount of boron in a diamond increases so too does the strength of blue. Blue diamonds are almost always type II b, do not contain fluorescence and generally have few imperfections.
The Argyle mine produces another form of blue diamonds of the type, a type which are sometimes classified as violet by the GIA. These stones share qualities with their pink cousins in that they are generally included and have fluorescence.
Their colour is caused by the element impurity hydrogen present in their covalent bonding. GIA grades blue diamonds in terms of tone and saturation. As tone and saturation increase rarity goes up as well as value.
Starting from low tone and saturation and increasing GIA grades blue diamonds as light, fancy light, fancy, fancy intense / fancy deep, and fancy vivid. All categories are very rare and small or large sizes.
Blue diamonds are modified by a violet, green, or grey. When reading a GIA certificate of a multi hue colour stone the last hue describes the dominant colour of the diamond. In other words a Fancy Grey Blue diamond shows more blue colour than a Fancy Blue Grey. Grey exists when the Saturation of a diamond is not strong enough to mask the tone and as more grey is visible the less valuable the diamond generally is.
Exceptions do exist however, demonstrated by the Wittelsbach and the Hope diamond, both are classified as having grey in their colour but are considered to be two of the most valuable diamonds ever mined.
Green can also be a modifier of blue diamonds. Green is present when a diamond has both boron in its atomic structure and has been near radiation for a period of its formation. Green as a modifier usually decreases the value of a diamond but not as significant as grey does.
Once a stone is close to pure green however the value starts to increase once again. Violet can also be a modifier of blue diamonds. Violet Blue diamonds are so rare however that the total amount of stones can be counted on ones fingers.