Judge a sapphire in two lights, not one

Sapphire colour is not a fixed property of the stone, it is what the stone does with the light it is given. Daylight is broad and even. Domestic and retail lighting is narrower and usually warmer, and it can flatter or flatten the same sapphire dramatically.

This is why we show a stone in more than one light before anyone is asked to decide. A blue that reads clean and open by a window can pick up a grey cast under a warm downlight. A stone that looks unremarkable indoors can lift considerably in daylight. Neither reading is the true one; both are true, and you will own the stone in both.

The practical habit is simple. Look at the stone near a window, away from direct sun. Then look at it again under the light you actually live with. If a seller will only show you a sapphire under one carefully chosen source, that is worth noticing.

It matters most for stones near a boundary: blues that carry a violet or green secondary, teals that sit between two hues, and any stone dark enough that tone is doing the work. Those are exactly the sapphires that reward a second look.