Coloured stones are not graded the way diamonds are. There is no universal scale, and two stones with the same description can differ in price by a multiple. Three things account for most of it.
Colour
Not just the hue, but its saturation and how evenly it sits across the stone. A ruby described as "pigeon's blood" or a sapphire as "royal blue" is being placed in a narrow, premium band of colour, and those terms appear on the certificate, not just in conversation.
Origin
Origin carries a premium because certain deposits have historically produced a particular colour: Burma for ruby, Kashmir and Ceylon for sapphire, Colombia for emerald. A stone from a celebrated source costs more than a visually identical one from elsewhere. Whether that premium is worth paying is a personal decision, and we will tell you plainly when you are paying for the name rather than the stone.
Treatment
Most coloured stones are treated, and this is normal and disclosed. Heat is standard for ruby and sapphire. Emeralds are almost always oiled. The question is how much. An untreated stone with a certificate confirming it commands a substantial premium precisely because it is uncommon.
One practical note
Colour shifts under different lighting far more visibly than in a diamond. Always look at a coloured stone in daylight and under warm indoor light before deciding.
