What Actually Changes a Sapphire's Price
Buyers almost always open with carat weight. It is the easiest number to hold in your head, and it is the one that matters least out of the four things that set a price.
Colour does most of the work
A well saturated 2 ct blue sapphire will usually cost more than a washed out 4 ct one, and sometimes several times more. Saturation and tone are what your eye actually reads across a room, and they are what a buyer remembers a month after the parcel arrives.
Within blue, the market pays a premium for a strong even blue that holds its colour under artificial light. Stones that go grey or inky under a lamp are cheaper for a reason, and you will notice it every evening you wear it.
Treatment doubles or halves everything
Most sapphire on the market has been heated. There is nothing wrong with that, and an honestly sold heated stone is good value.
The gap is the point. An unheated stone of the same size and colour can be worth several times a heated one, which gives every seller in the chain a reason to be vague. This is why an unheated claim without a laboratory report is not a discount, it is a risk.
Cut is invisible until you see two side by side
Cutters in Sri Lanka are usually paid on the weight they retain, so there is constant pressure to keep a stone heavy rather than to make it bright. A deep belly holds weight and hides light. A window in the middle means you can read newsprint through the stone.
Two stones at the same weight, colour and treatment can differ by a third in price purely on how well they were cut, and that difference is obvious the moment they sit next to each other.
Size matters, at the thresholds
Price per carat does not climb smoothly. It steps at round numbers, because that is where demand sits. A stone just over 3 ct costs noticeably more per carat than one just under, for reasons that have nothing to do with the stone.
If you are flexible, buying just below a threshold is the single easiest way to get more gemstone for the money. A 2.85 ct stone and a 3.05 ct stone look identical on a hand.
What this means when you are buying
Ask for the treatment status and the report before you ask about weight. Then ask to see the stone on video under one light, tilted, so you can judge the cut and watch what the colour does. Then talk about size.
Buyers who work in that order end up with better stones for the same money, every time.