"Rainbow Fire" and Projection: How Gem Cuts Affect Visual Impact

Gemstone fire, technically known as dispersion, occurs when white light enters a faceted stone and is split into its component spectral colours. In projection jewellery, balancing this optical phenomenon with image clarity is critical; excessive faceting on the central projection lens causes light scattering, which blurs the hidden image. Therefore, the most effective designs utilise a smooth, plano-convex centre stone for projection, typically surrounded by high-dispersion halo stones for visual brilliance.

The Dance of Light and Memory

There is a moment I love when I hand a finished piece to a client in our studio. They take it to the window, and two things happen simultaneously. First, the sunlight hits the surrounding stones, sending tiny flashes of red, blue, and violet dancing across the walls. Second, they lift the pendant to their eye, and the world goes quiet as they peer into the central lens.

It is a delicate balance. We want the jewellery to be alive with light—that "Rainbow Fire"—but we also need the centre to be a still, clear pool of water for the memory to rest in.

I often see confusion about why some projection necklaces sparkle aggressively but have blurry photos, while others look sleek and offer high-definition projection. The answer lies in the geometry of the cut. As a gemologist, I view every facet as a mirror and a window. If we place the wrong window in front of your photograph, the view is lost.

1. The Physics of "Fire" (Dispersion)

To understand why your necklace sparkles, we have to talk about how light behaves when it hits a solid object.

Key Concept: Dispersion

When white light passes through a transparent material, it slows down and bends (refracts). Different colours bend at different angles—blue bends more than red.

High-quality gemstones act like prisms, separating these colours. The wider the separation, the more "Fire" or rainbow flashes you see.

• Diamond Dispersion: 0.044
• Cubic Zirconia (CZ) Dispersion: 0.060 (Higher than diamond)
• Moissanite Dispersion: 0.104 (The "Fire King")

At PhilU, we select materials specifically for this property. We use 5A Cubic Zirconia because its dispersion (0.060) is actually higher than a natural diamond. This means that while the central lens is doing the heavy lifting of projecting your photo, the surrounding halo is working overtime to catch the eye with spectral colour.

2. The Conflict: Facets vs. The Lens

Here is the optical challenge: Sparkle requires facets. Projection requires smoothness.

The Faceted Trap

Some manufacturers try to make the central projection stone itself look like a diamond by covering it in facets (flat, polished faces).

• The Look: It sparkles beautifully.
• The Projection: It is a disaster. Each facet acts as a mini-prism, deflecting the light path. When you try to project the image, the light is scattered in twenty different directions. The photo appears "shattered" or segmented, like looking through a kaleidoscope or a fly’s eye.

The Plano-Convex Solution

To get a crisp, HD image, the central stone must be a Plano-Convex Lens.

• The Shape: A smooth, curved dome on the front (convex) and a flat back (plano).
• The Physics: This smooth curve bends light uniformly towards a single focal point.
• The Trade-off: A smooth dome doesn't sparkle as much as a faceted diamond. It looks more like a polished cabochon or a drop of water.

This is why we engineer our designs with a "Halo Architecture." We keep the central lens optically pure (smooth) for the sake of the memory, and we surround it with a ring of brilliant-cut stones to provide the "Rainbow Fire."

3. Comparison: Selecting the Right Stone for the Halo

If the centre stone is the "Projector," the surrounding stones are the "Frame." The material you choose for this frame drastically changes the visual impact of the piece.

Gemstone Material	Refractive Index (RI)	Dispersion (Fire)	Visual Effect	PhilU Recommendation
Crystal / Glass	1.50	0.010	Flat, dull shine. No rainbows.	Avoid. Looks like costume jewellery.
Natural Diamond	2.42	0.044	Sophisticated, white brilliance.	Classic. Elegant but expensive.
5A Cubic Zirconia	2.15	0.060	Intense rainbows.	Best Value. Matches the optical weight of the lens.
Moissanite	2.65	0.104	"Disco Ball" effect. Extreme fire.	Statement. For maximum attention.

4. How to Evaluate the Cut

When you are looking at a projection necklace, pay attention to how the central stone is cut.

Step-by-Step: The Visual Audit

1. Check the Dome: Run your finger over the centre stone. It should feel smooth and rounded. If you feel sharp edges or flat facets across the entire face, the projection will likely be distorted.
2. The "Northern Star" Reflection: Hold the pendant under a single light bulb. A high-quality smooth lens will show a single, sharp reflection of the light source on its surface. A faceted stone will show multiple, scattered reflections.
3. The Halo Transition: Look at where the lens meets the metal or the halo stones. At PhilU, we use Sterling Silver (925) prong settings that sit low. This allows maximum light to enter the side of the surrounding stones, igniting that fire, without interfering with the central optical path.

5. Moissanite: Too Much of a Good Thing?

We often get asked about making the entire projection lens out of Moissanite. While Moissanite is incredibly hard (9.25 Mohs) and fiery, it has a property called Double Refraction.

When light enters Moissanite, it splits into two separate rays.

• For Jewellery: This creates twice the sparkle.
• For Projection: This creates a "ghosting" effect. You might see two slightly overlapping images of your loved one.

This is why, as physicists, we prefer Cubic Zirconia for the projection lens itself. It is Singly Refractive (like diamond), meaning it keeps the image sharp and singular, while offering enough durability to last a lifetime.

Summary

Designing these pieces is about respect. We respect the laws of physics, and we respect the emotion of the memory.

We accept that the central stone must be smooth to do its job—to tell the story clearly. And we let the surrounding stones do the dancing. When you wear a piece that balances these elements correctly, you get the best of both worlds: a fire that catches the eye, and a secret image that captures the heart.