Guides · Design

Backlit onyx: how to design it properly, which LEDs to use, and why the light must not heat the stone

In short. Onyx can be backlit because it is translucent, but no two slabs light up the same way: the slab has to be chosen against the light, before it is cut. The right light is warm (2700-3000 K), dimmable and evenly diffused, with no bright spots. Above all, the light source must not heat the stone: remote drivers, a ventilated cavity and access for maintenance matter as much as the beauty of the slab.

What exactly do we mean by onyx?

The name causes confusion. Mineralogically, onyx is a variety of agate — banded chalcedony, a hard silica stone (6.5-7 on the Mohs scale). What the building and furniture trades call onyx — the honey, green, white and pink slabs used for bars, walls and lamps — is almost always banded calcite or aragonite, a calcium carbonate. It is far softer than chalcedony and it behaves like a marble: it scratches more easily and it reacts with acids.

That has two practical consequences. First, care is the same as for marble, with the same caution towards lemon, vinegar and descaling cleaners, as we explain in our guide Marble stains and etching. Second, the banded structure — its veins, fissures and crystals — is exactly what the light reveals. Backlit, a slab is read through its thickness, flaws included.

Why do some slabs glow and others stay dark?

Translucency varies enormously from one block to the next, and even within a single slab. Some onyx lets through a full, golden light; other slabs, beautiful in daylight, stay dull and show only a blurred outline of the source. There is no way to tell from a photograph: slabs are chosen against the light, in front of a source, before cutting.

That is also when you see what reflected light hides: filled natural fissures, more opaque areas, dark veins that turn into hard lines. It is the moment to plan the cuts so that the pattern flows from one piece to the next.

Thickness plays its part. Backlit applications usually use thin slabs, between 10 and 20 mm: the thicker the slab, the more light it holds back. A thinner slab, however, is more fragile — which brings us to support.

Which LEDs, and what kind of light?

The choice is less technical than it looks and more decisive than most people expect. Three rules almost always apply.

Warm light. Onyx comes in honey, amber and green tones; a colour temperature between 2700 and 3000 K brings out its golden veining. Cool light tends to flatten the colours and gives the stone a flat, almost signage-like look.

Dimmable light. A bar, a wall or a lamp does not live at one intensity: by day the stone should remain stone, in the evening it can become the main light in the room. Being able to dim it is what makes onyx a furnishing material rather than a fixed stage effect.

Even light. Point-source LED strips mounted too close draw a row of bright dots across the stone. They are avoided with diffuse-emission LED panels, or with more distance and an opal diffuser between the sources and the slab. When an installation uses several panels, the LEDs should come from the same batch, because slightly different tints become obvious across a large pale surface.

Does backlighting heat the stone?

It should not, and it is the point we insist on most. LEDs produce far less heat than the lamps of the past, but there is always some heat, above all in the drivers and in high-density strips. Shut in an airless cavity, that heat builds up.

Stone that warms unevenly works: it expands where it is warm and stays put where it is cool. Adhesives and backing materials warm up too, and some of them tend to yellow with heat and light. Over the years the result can be a yellower light near the sources and an unnecessary risk to the slab.

So the rule is simple: the light illuminates, it does not heat. Drivers go outside the cavity, somewhere ventilated and reachable; the sources should run well below their maximum output; the cavity must breathe. Touched after a few hours of operation, the stone should not feel warm.

How far should the LEDs be from the slab?

It depends on the source. With diffuse LED panels, manufacturers typically indicate a gap of about 2.5 to 4 cm (1 to 1.5 inches); point-source strips need more depth, or an intermediate diffuser, or the dots stay visible. The available depth has to be decided when the cabinet or wall is designed, not afterwards: a bar front that leaves only a centimetre of space forces the wrong light source.

How do you support a thin slab without casting shadows?

A thin onyx slab needs support, especially over large surfaces. We do not use resin to reinforce our slabs: small surfaces need no reinforcement, while on large ones it helps carry the material. For backlit onyx we use a backing mesh that does not show through and lets the full light pass, a material also used by some other makers who work stone as we do. No added glass: the slab stays stone, lighter and brighter.

The test is simple: the support must carry the stone without turning into a visible pattern when the light comes on, and without taking away any brightness.

Can backlit onyx go in a bathroom or a shower?

Yes, with two separate precautions. The first concerns the electrics. Bathrooms fall under IEC 60364-7-701 (in Italy, CEI 64-8 section 701; in the UK, BS 7671 section 701), which divides the room into zones around the bath and shower and sets a water-protection rating for each — IPX7 inside the bath or shower tray, IPX4 in the zones around it, IPX5 where water jets can reach — along with 30 mA residual-current protection and, where safety extra-low-voltage circuits are used, a power source installed outside zones 0, 1 and 2. Local rules vary: this is a matter for the designer and the electrician, and it should be settled before the stone is chosen.

The second concerns the stone: onyx dislikes hard water and the acidic products used to remove limescale. In a shower it needs more careful upkeep than ceramic, and it is best to know that from the start.

What happens when, ten years from now, a light source fails?

It is the question almost nobody asks at design stage and everybody asks afterwards. Well-designed backlighting can be serviced without removing the stone: a removable rear panel, an accessible service void, reachable drivers. A slab permanently bonded in front of the lights turns a component swap into a building job.

How we work

At Stoneform, backlit onyx is first of all a design question. We look at every slab against the light before cutting, to guarantee the highest perfection in the finished piece; we work the stone in our own factory and design light, support and serviceability together. We have made many backlit onyx pieces, for hotels and for private residences alike. LuxOnyx, the modular system designed by Giuseppe Fallacara and Andrea Sgherza, sums it up: hollowed onyx modules lit from within and joined by copper fittings, a successful exercise that shows the level of perfection we bring to working any stone. Our other luminous pieces are in the Sculpture and lighting collection.

For bars, walls and lobbies in hotels and residences, our Contract page explains how we follow projects. Stoneform and Brianform are two divisions of the same company: together they offer a complete service, from stone to furniture, with no intermediaries and therefore at lower cost. Production is in-house, so there are no unexpected mark-ups. For a quotation, simply write to us with dimensions and photos of the space: we reply within 48 hours.

Frequently asked questions

Does backlit onyx turn yellow?

The stone does not; some adhesives and backings can yellow over time, especially when exposed to heat. Light sources that stay cool and materials chosen for use with light avoid the problem.

What colour temperature should I choose?

Warm light, between 2700 and 3000 K, dimmable. It brings out the amber and golden tones of onyx.

Can marble be backlit too?

Only some white marbles, cut very thin, let light through, with a softer effect. Onyx remains the most luminous material.

How thick should the slab be?

Usually between 10 and 20 mm: thinner and the slab becomes fragile, thicker and it holds back too much light. For table and counter tops see also our guide How thick should a marble table top be.

How much does it cost?

It depends on the slab, the dimensions and the lighting and support solution. With dimensions and photos we prepare a quotation within 48 hours.

Ask for a quotation See LuxOnyx

The distinction between mineralogical onyx (banded chalcedony, 6.5-7 Mohs) and commercial onyx (banded calcite or aragonite) follows current mineralogical literature. The reference thicknesses for backlit applications and the gaps quoted for diffuse LED panels are figures declared by the makers of the materials and of the light sources, not standards: they should be checked against the product chosen. The bathroom requirements are those of IEC 60364-7-701 (CEI 64-8 section 701 in Italy, BS 7671 section 701 in the UK); local rules vary and remain the responsibility of the designer and the electrical installer.