Why darker is not better

Buyers trained on sapphire and emerald reach for the deepest stone in the tray. In tourmaline that instinct costs money.

Colour in a gemstone is three separate things, and conflating them is the most common and most expensive mistake an inexperienced buyer makes. Hue is which colour it is. Saturation is how free that colour is of grey. Tone is how light or dark it is.

In sapphire and emerald, tone and value move together up to a point — a pale sapphire is a cheap sapphire, so the eye learns that deeper means better. Tourmaline punishes that habit, because a great deal of tourmaline is naturally dark, and darkness in this species is usually the enemy of everything that makes it worth buying.

Watch what happens as tone increases. Past medium-dark, the stone stops returning light through the pavilion and starts absorbing it. The colour deepens on paper and dies in the hand. This is most visible in green and blue material: a forest-green verdelite photographs impressively and reads black on the finger.

The peak of value in tourmaline sits at high saturation with medium to medium-light tone. That is where a rubellite still reads red rather than maroon, where an indicolite is still blue rather than ink, and where a cuprian stone still glows. The entire premium of Paraíba-type material depends on this: the neon effect only exists because the stone is light enough to let light through.

There is a related trick worth knowing. A cutter working with pale rough can force apparent colour by cutting the stone very deep, so light has a longer path through the material. It works in the tray and fails in a mounting, where the extra depth has to be hidden and the stone looks small for its weight. Always look at a tourmaline from the side. If the pavilion is unusually deep, the colour you are admiring is a manufacturing decision, not a property of the material.

The other silent value killer is grey. Almost nothing in a photograph gives it away, and almost every ordinary tourmaline has it. The way to see it is comparison: put a fine chrome tourmaline next to an ordinary green one and the grey-olive component in the second becomes obvious immediately. Alone, it looks perfectly good.

Which leads to the only practical advice worth giving. Judge colour against other stones, not against your memory, and judge it in more than one light. Fine rubellite holds its red under incandescent light; ordinary material turns brown and no photograph will ever tell you which one you are looking at.