Fluorescence IRL: Why Some Stones Glow on Cloudy Days in London but Not in LA

Fluorescence IRL: Why Some Stones Glow on Cloudy Days in London but Not in LA

Walk across Trafalgar Square on an overcast afternoon and a ruby or certain diamonds can appear to “glow.” Stand under the bright sun in Los Angeles and the same stone looks ordinary. That difference is not superstition. It’s physics, chemistry, and human vision working together. Here’s why some gems fluoresce more on cloudy days in London than they do in sunny LA — and what that means when you buy or wear jewelry.

What fluorescence is, in plain terms

Fluorescence happens when a material absorbs short-wavelength light (usually ultraviolet or near‑UV/blue) and re-emits part of that energy as visible light. The emitted color depends on the atoms or defects inside the crystal. The incoming UV excites electrons; when they relax, they emit visible photons. The eye sees that emitted light as a glow that can appear stronger than the gem’s normal reflected color under certain lighting.

What excites gemstones — the important wavelengths

Most gem fluorescence is driven by UV-A and near-UV/blue light roughly in the 320–420 nm range. The atmosphere filters out UV-C and most short UV-B, so the ground-level solar spectrum that matters is UV-A and visible blue. Labs test fluorescence with lamps at 365 nm (long-wave) and 254 nm (short-wave). In practice, natural daylight and skylight contain enough near-UV to make many stones glow.

Which impurities make stones glow — and examples

• Rubies and red spinels: Chromium (Cr3+) is the activator. Rubies show bright red emission — the same atomic transition that produces the 694 nm ruby laser line. A 1.0 ct ruby (roughly 6.5 × 4.5 mm) will often appear more vivid under UV because Cr3+ radiates efficiently.
• Diamonds: Blue fluorescence in diamonds usually comes from nitrogen-related defects and vacancy centers. Many gem-quality diamonds are graded as having “none,” “faint,” “medium,” “strong,” or “very strong” fluorescence under 365 nm light.
• Fluorite: Often one of the most strongly fluorescent minerals, activated by rare-earths or Eu2+, showing blue, green or purple glows depending on the impurity.
• Garnets and some tourmalines: Manganese (Mn2+) or trace rare-earths can cause orange-red or yellow fluorescence.
• Opals: Play-of-color is different from fluorescence, but some opals contain organics or trace elements that fluoresce under UV.

Why cloudy London skies can make glow — three concrete reasons

• Diffused light reduces surface glare. On a sunny day, direct sunlight causes strong specular highlights and reflections on a polished stone. Those bright reflections overwhelm the relatively faint fluorescent emission. Clouds scatter direct light into many directions. That lowers bright reflections and lets the eye pick up weaker fluorescent light.
• Sky-scattered short wavelengths are still present. Thin or broken cloud layers pass a lot of near-UV and blue light while scattering it. The result is lower overall brightness but a higher fraction of scattered blue/near-UV hitting the stone from many angles. That increases excitation across the gem’s surface and spreads the emitted light, making the glow more visible.
• Human vision adapts. In lower ambient light the pupil dilates and our rods and cones become more sensitive to subtle color shifts. Under overcast light your eye is literally better at noticing small amounts of extra visible emission from fluorescence than it is in glaring sun.

Why LA sun might hide the same fluorescence

Los Angeles gets strong direct sunlight with a high absolute UV intensity. That would seem to favor fluorescence, but two effects work the other way:

• Specular washout. Bright direct light produces intense reflections and highlights. Those swamp fluorescent emission that’s only a fraction of the reflected visible light. A stone in full sun often looks “flat” because the fluorescent contribution is a small percentage of total light reaching your eye.
• Angular lighting matters. Fluorescence is emitted isotropically (in all directions) but it’s most noticeable when bright reflected light is minimized and when excitation reaches internal areas of the stone. In direct sunlight the strongest illumination is directional and can leave parts of the stone in deep shadow while other parts glare — an arrangement that doesn’t favor evenly distributed fluorescent glow.

Geography and atmosphere: yes, but not the whole story

Latitude, altitude, ozone layer and pollution change the absolute UV dose. LA typically has higher UV irradiance than London. But absolute UV is only one factor. Whether you see fluorescence depends on the mix of direct vs diffuse light, cloud thickness, and how your eye perceives contrast. A thin, bright overcast day in London can present a more fluorescence‑friendly mix than a high‑sun LA afternoon that simply overwhelms the effect.

Treatments, size, and practical examples

• Size matters. A 0.25 ct diamond (~4.1 mm diameter) with faint fluorescence will rarely show any glow outdoors. A 2 ct diamond (~8.2 mm) with strong fluorescence is far more likely to look different under the same light.
• Treatments can change things. Oil-fill in emeralds can mute or alter fluorescence. Coatings on diamonds or glass-filled rubies can add or remove fluorescence. Always ask for treatment disclosure.
• Real-world example: a 1 ct ruby ring left in a London courtyard under a sheet cloud will often look richer and slightly luminous because Cr3+ is emitting red light across the stone. In LA the same ruby under direct sun will look extremely bright, but the specular reflections make fluorescence less obvious.

How to test and what to ask for

• Bring a 365 nm long-wave UV torch when buying colored stones. Watch for the characteristic color of emission (red for ruby, blue for many diamonds, etc.).
• View stones both under direct sun and in bright overcast sky. If the dealer is local, check in both conditions or ask for photos taken in diffused daylight.
• For diamonds, request the lab report that notes fluorescence strength. For colored stones, ask about trace elements or treatments known to affect fluorescence.
• Consider wearing the piece. If you want that “glow” on cloudy days, test it on a walk. If you prefer consistent appearance in bright sun, make sure fluorescence won’t make the stone look hazy or change color in indoor lighting.

Bottom line

Fluorescence is a real, measurable effect that depends on the stone’s chemistry and on the light that excites it. Cloudy, diffuse skies can reveal glows that direct, intense sunlight hides. London’s typical overcast lightscape often favors seeing that glow, while bright LA sun can wash it out. When buying or evaluating gems, test in multiple lighting conditions and ask for lab details so you know whether the glow you like will be there when you wear the piece.