Fluorescence

Fluorescence is a visible glow set off by ultraviolet light. Daylight carries enough ultraviolet to do it. Chromium is the cause in ruby and red spinel. In low-iron rubies, classically from Mogok, red fluorescence adds a lit-from-within intensity that iron-rich stones cannot match; it is part of what the market pays for, and part of why the trade tied "pigeon's blood" to Burma in the first place. Iron kills it. Hold a Thai ruby of the same hue beside a Mogok stone in the sun and the Thai stone goes flat. Sapphire and emerald mostly show little. Diamond buyers learn that fluorescence is a discount; carry that instinct across into ruby and you will pay for the wrong stone.

On a lab report

To a laboratory, fluorescence is evidence. Nobody grades it. A gemmologist puts the stone under long-wave and then short-wave ultraviolet and reads the reaction: low-iron Burmese ruby glows a strong red, iron-rich Thai ruby stays dull, and that difference is part of the origin opinion. A chalky bluish reaction under short-wave in a sapphire has been read as a sign of heating for decades. Flame-fusion synthetic ruby, with no iron, can glow harder than the real thing, which is one reason a strong glow on its own proves nothing. Almost none of this reaches the printed report. It sits behind the origin line and the treatment line; a few houses add a note in the comments, most say nothing. Absence from the paper does not mean absence from the stone.

Ask to see a fluorescent stone in plain daylight, where the effect actually lives. A jeweller's lamp flatters everything. Put a Mogok ruby by a window next to a Thai stone of the same grade and the difference teaches itself in a minute. That glow in the sun is what you are paying for, so make sure it is there before the money moves.