What is onyx stone, and is it practical for a bathtub or basin?

Is bathroom onyx the same as onyx the gemstone?

No. True onyx is a variety of chalcedony: fine quartz and moganite in straight parallel bands, translucent, at Mohs 6.5–7. The stone in interior onyx is calcium carbonate. Wikipedia notes that most carved onyx sold today is the softer carbonate type, and that names such as Mexican onyx or cave onyx are misleading.

What gives onyx its banding and translucency?

Deposits grow as layers of prismatic crystals, separated by thin microcrystalline bands. Minor mineral substances deposited during formation account for color in natural stone generally, and in onyx they show up as stripes and clouds. The Eifel Aqueduct deposit was long called onyx marble in England, although it is neither onyx nor marble.

Light-colored calcite alabaster is translucent, and the same applies to pale onyx in thin sections. Darker bands and thick walls block more light, so no two blocks transmit the same amount.

How does onyx behave with water, heat and acids?

Acids attack it first. Calcite dissolves in dilute acid, so lemon, vinegar and acidic cleaners dull a polished surface quickly, and a dull spot on a smooth, richly colored stone is easy to see. Water is a slow effect: alabaster-type stone dissolves slightly in water, so standing water and wet soap residue should not sit on it for days.

At about Mohs 3, onyx scratches with grit and chips at corners. The heat of bathwater is not a concern, but sudden knocks are, so rims and thin spouts need care in handling.

Can onyx be backlit, and does it glow in a tub?

Pale onyx passes light through thin sections, which is the basis of the glowing look people want. A solid tub has walls measured in inches, so we make no promise of a glow; the effect depends on the block's color, the wall thickness and the lighting behind it. Ask for a sample held to a light before choosing.

Where does onyx work, and where does it not?

• Vessel basin: the strongest fit, since it sees brief contact and can be wiped dry.
• Freestanding tub: possible with careful use; the onyx bathtub guide covers fit, weight and cleaning.
• Vanity top: pretty, but products and drips will print on it.
• Avoid: outdoor use, hard-water rooms with no one drying surfaces, and homes where cleaning is handled by a service using general-purpose sprays.

A tub in onyx weighs about the same as marble in our weight model at 168 lb/ft³, so the stone bathtub weight guide applies. Check floor loads with a structural engineer.

How do you care for an onyx piece?

1. Dry the stone after use.
2. Clean with a pH-neutral stone cleaner on a soft cloth.
3. Keep acids, bleach blends and abrasive pads away from it.
4. Set bottles on a tray so rings do not form.
5. Use a sealer made for natural stone, and have a professional handle chips or etching.

More detail is in the stone bathtub care guide.

How does onyx compare with marble and travertine?

Onyx and marble are both calcium carbonate, so acid sensitivity is shared. Onyx adds translucency and bolder banding at the price of more care. Marble is the more forgiving working stone.

Compared with travertine, onyx is smoother and more colorful, with a more polished look. Travertine is the better choice when you want a quiet, warm tone and a lived-in surface.

What does Pietra Fonte make in onyx?

Pietra Fonte carves made-to-order onyx bathtubs and onyx basins in white, green, pink and striped white. Each is cut from a single block, specified with the buyer, and inspected with photos or video before shipping. Every quote gives exact dimensions and weight.

Block selection matters more with onyx than with most stones, so start with a sample or talk to the trade desk.