Why Can Gold Be Red? Nanogold, the Lycurgus Cup and the Science of Color

Gold looks yellow in a ring, but when it is divided into particles only a few billionths of a meter across, it can glow ruby red or purple. That happens because tiny gold particles interact with light in a completely different way from solid metal, through an effect called surface plasmon resonance. Whether you are reading this in Miami under a bright sky or anywhere else, the gold on your wrist and the red in a piece of historic ruby glass are the same element, simply working at different scales.

Why does solid gold look yellow?

Most metals, like silver and platinum, reflect nearly all colors of visible light about equally, so they look white or gray. Gold is different. Its electrons absorb part of the blue end of the spectrum and reflect the rest, so what reaches your eye is warm yellow. Scientists explain this unusual absorption with relativity: gold's heavy nucleus makes its inner electrons move so fast that relativistic effects shift its energy levels, bringing the absorption into the visible range.

That warm color is the starting point for every gold alloy. Add copper and the metal turns rosier; add silver or palladium and it pales. If you want the full chemistry, our article on gold alloys and yellow, rose and white gold explains each recipe.

Our Wavy Diamond Bracelet is made in 18K recycled gold, offered in yellow, rose or white, with ethically sourced natural diamonds. Its rose version gets its blush from copper in the alloy, a different mechanism from the red you are about to meet.

What is nanogold?

Nanogold, also called colloidal gold, is gold divided into particles measured in nanometers. A nanometer is one billionth of a meter, so these particles are far smaller than the wavelength of visible light. Suspended in water or trapped in glass, they do not look like metal at all. Small, well separated particles make a liquid or glass look ruby red. Larger particles, or particles that clump together, shift the color toward purple and blue.

Why do gold nanoparticles look red?

Light is an electromagnetic wave. When it hits a gold particle much smaller than its wavelength, the wave's oscillating electric field pushes the particle's free electrons back and forth together, like water sloshing in a small bowl. At one particular frequency, that collective oscillation resonates. This is the surface plasmon resonance.

• For small gold nanoparticles, the resonance falls in the green part of the spectrum.
• Green light is strongly absorbed and scattered.
• The light that passes through is rich in red, so the glass or liquid looks red.
• Change the particle size, shape or spacing, and the resonance moves, which changes the color.

The science of waves sits at the heart of this, which is why it feels close to the Wavy collection, drawn from the sine and cosine curves Margaret studied during her years in physics. A wave of light meets a wave of electrons, and color is the result.

The Wavy Bow Diamond Hoops, in 18K recycled gold with approximately 1.85 ct of ethically sourced natural diamonds, are that idea in solid metal: a continuous wave of gold tied into a bow.

What is the Lycurgus Cup?

The most famous example of ancient nanogold is the Lycurgus Cup, a Roman glass cup from the 4th century AD, now in the British Museum. In ordinary reflected light it looks jade green. Lit from inside, it glows red. As Smithsonian Magazine has described, scientists found the glass contains tiny particles of gold and silver, and the way those particles absorb and scatter light creates the dramatic change. The Roman glassmakers almost certainly did not understand the physics, but they mastered the effect.

How did nanogold color glass and porcelain?

For centuries, glassmakers used gold to make deep red glass, often called gold ruby glass or cranberry glass. In the 17th century, a pigment called Purple of Cassius, made from gold and tin, spread through glass and porcelain workshops in Europe. In 1857, Michael Faraday studied colloidal gold at the Royal Institution in London and recognized that its color came from gold in an extremely finely divided state. It is one of the founding moments of what we now call nanoscience.

Where is nanogold used today?

Gold nanoparticles are still at work, now in medicine and technology:

• Rapid tests: the colored line on many lateral flow tests, such as home pregnancy tests, often comes from gold nanoparticles.
• Research: scientists study gold nanoparticles for sensing and imaging because their color changes predictably with their surroundings.
• Art and conservation: understanding nanogold helps restorers analyze historic glass and ceramics.

It is a lovely reminder that the most precious metal in jewelry is also one of the most useful materials in a laboratory, as we explored in why gold does not tarnish.

Frequently asked questions

Is red gold jewelry made with nanoparticles?
No. Red and rose gold jewelry gets its color from copper alloyed with gold. Nanogold red appears only when gold is divided into tiny particles inside glass or liquid.

Is the gold in a ring the same element as nanogold?
Yes. Both are gold. The difference is scale: a ring is solid metal, while nanogold is made of particles billionths of a meter across.

How long does delivery to Miami take?
Margaret Krinetz Fine Jewelry, handmade in Tel Aviv and shipped to Miami, makes every piece to order in 4 to 6 weeks, then ships with DHL Express in 3 to 5 business days, free on orders over $1,500.

The same element can be a yellow ring, a red glass, a purple glaze. Choose the gold that tells your story, and explore our handmade gold bracelets.