The glow which 25-35% of diamonds emit when they are exposed to UV light – from the sun or fluorescent lamps. Fluorescence is perceived as bluish rays from the stone. Fluorescence is graded from None up to Very Strong. In certain circumstances the level of fluorescence can affect the price of a stone; in diamonds which appear colourless – graded D-F – it can lower the price whereas in slightly coloured diamonds – J-M, the bluish tinge can balance out the faint yellow colour of the stone.