Blue

The elements of boron may also be substituted within a diamond during its formation. Boron absorbs red light, hence in the absence of nitrogen, diamonds containing boron are blue in colour. An example of a diamond containing boron is the famous Blue Hope diamond. Diamonds containing boron also exhibit unusual electrical properties and are semi-conductive in nature. Hydrogen is another impurity that, in high quantities, can cause grey or blue colouring in diamonds. However, these diamonds are not semi-conducting.