What is fluorescence in a lab-grown diamond?

Fluorescence in a lab-grown diamond is a visible glow, most often blue, that some stones emit when exposed to ultraviolet light. IGI grades fluorescence intensity on a scale from None to Very Strong; the majority of lab-grown diamonds fall at None or Faint. A low or medium fluorescence grade has no measurable effect on how a diamond looks in daylight or indoor lighting. Fluorescence is neither a defect nor a bonus grade; it is one characteristic among several. When comparing two otherwise identical stones, let the fluorescence grade inform rather than decide your choice.