Onyx vs marble: what's the difference for a bath or sink?

How do onyx and marble form?

Marble forms when limestone is subjected to the heat and pressure of metamorphism, and its calcite recrystallizes. The result is a dense rock of interlocking crystals, usually with veins of other minerals such as clay, mica or pyrite.

Banded calcite onyx forms differently. One stone supplier describes it as the gradual layering of calcite in water-rich environments, and another says dripping and evaporating water deposits the layers. Those layers become the parallel bands and the clear, glassy depth that marble lacks.

Is onyx harder or softer than marble?

They are equal on paper. Calcite defines 3 on the Mohs scale, and both stones are mostly calcite. For comparison, granite is mostly quartz and feldspar, which rate 7 and 6.

In practice onyx behaves as the more delicate stone. One stone supplier says it scratches easily and that extracting good blocks is delicate, with a risk of cracking. That is a fabrication and handling point as much as a daily-use one, and it is why thick, supported construction matters for onyx basins and tubs.

Why is onyx translucent and marble not?

Onyx is fine-grained, banded calcite with few internal boundaries to scatter light, so light passes through thin sections. Marble's interlocking crystals and mineral veining scatter and absorb it, so the stone reads as opaque. Pale onyx glows when lit from behind; the same light behind marble stays on the surface.

Translucency falls off with thickness. A thin panel glows; a thick-walled carved piece shows depth and color but will not light up on its own.

Does onyx stain and etch more than marble?

Both etch. Calcite dissolves in acid, so toothpaste, mouthwash, many bathroom cleaners, vinegar and citrus can dull a polished surface on either stone. One fabrication guide notes that sealers do nothing for etching and only slow staining, and that minor etching can sometimes be buffed out while deeper etching needs professional re-honing.

Onyx is more demanding in two ways. It is usually light-colored and translucent, so stains and dull marks are easier to see. One stone vendor also advises resealing onyx every 6 to 12 months and describes it as porous and easy to stain or etch with acidic or colored liquids. Marble vanity guidance is similar, at 6 to 12 months depending on use. Our stone care guide covers daily cleaning.

How do onyx and marble compare side by side?

Can onyx be used in a bathroom?

Yes. Onyx is common for vanity fronts, vessel basins and wall panels, and bathroom surfaces see less acid and abrasion than a kitchen counter. It needs a wipe-up habit: toothpaste, cosmetics and acidic cleaners should come off promptly, and ammonia and alcohol should be kept away from it.

It also needs a sealing schedule and sensible placement. One stone vendor recommends sealing every 6 to 12 months, and describes onyx as best for vertical applications and statement pieces. For a tub used daily or a basin that takes heavy use, marble is the lower-risk choice.

Which is better for a bath or sink?

For a tub or a basin used every day, marble is the lower-risk choice. It tolerates handling, hides small marks in its veining, and is available in more colors. See marble bathtubs and marble basins.

Choose onyx when the stone itself is the feature: a primary-bath tub, a vessel basin in a powder room, or a vanity front. See onyx bathtubs and onyx basins. Ask the quote to state wall thickness, weight and how the piece is supported.

• Busy family bath, heavy daily use: marble
• Powder room or primary bath, visual focus: onyx
• Backlit or lit-from-within effect: onyx only
• Lowest sealing and care effort: marble in a honed finish