According to Hard as Rocks, a blue diamond is a colored diamond and a member of the carbon family that contains the element boron. A colored diamond is defined as a diamond that contains impurities or structural defects that cause coloration other than the most common yellow or brown hues.
When the color is strong enough, the diamond is referred to as a fancy colored diamond by the diamond trade. There are natural fancy colored diamonds and synthetic colored diamonds.
A blue diamond biblically symbolizes heavenly grace. The most famous blue diamond is the French Blue Diamond that became known as the Hope Diamond. In 1762 during the French Revolution, it was stolen from the French Crown Jewels.