Fluorescence: When exposed to long wave ultra-violet light, some diamonds will glow (display fluorescence). The amount of fluorescence will determine a diamond's value. Fluorescence is graded from None to Very Strong. Faint or moderate blue fluorescence does not usually affect a diamond's appearance, it can actually make lower coloured (yellowish) diamonds appear whiter in daylight. Very Strong fluorescence can give a diamond a slightly oily or cloudy surface appearance.