Why colour changes under different light

Anyone who looks at a gemstone first by the window and then under a warm lamp is often surprised: the colour looks different, sometimes slightly, sometimes markedly. This is not a flaw in the stone but physics.

Not all light is the same

Every light source has its own spectrum, meaning its own distribution of wavelengths. Daylight contains a lot of blue and green; incandescent light and many warm LED lamps contain more red and yellow. A gemstone can only return or transmit the colours that are present in the light in the first place.

What the stone does with it

Colouring elements such as chromium, iron, titanium, vanadium or copper absorb particular wavelengths. What remains is what we perceive as colour. If a stone transmits red and green in similar strength, the light source decides which part dominates.

Colour change in alexandrite

In alexandrite this effect is particularly strong: green to bluish green in daylight, red to purplish red under incandescent light. The cause is chromium in chrysoberyl, which creates an absorption band in the yellow. Some sapphires, garnets and spinels also show a colour change, usually weaker. The change always affects the whole stone, not individual zones.

The eye plays a part too

Our perception of colour adapts to its surroundings. A stone in front of dark velvet looks different from one on white paper. That is why laboratories judge colour under standardised lighting against a neutral background.

What this means for photographs

A photograph shows the stone under a single lighting situation, further influenced by camera and screen. It cannot replace looking at the stone. Useful images state their light source and, for colour-change stones, ideally show two views under different sources.

In brief

Colour arises from the interplay of light source, absorption in the stone and perception. Always judge colour under known lighting conditions and, if possible, by direct comparison. Photographs help but never replace looking at the stone itself.

At NOBILITH

Dossier images are taken under controlled light and labelled accordingly. For colour-change stones we show both views, and during a consultation a stone can be viewed under different light sources.

Sources

• GIA: Gem Encyclopedia, alexandrite and colour change
• Nassau, K.: The Physics and Chemistry of Color