Short answer
Stone colour is the colour of its minerals and of what stains them. Granite is a coarse mix of crystals — pink or white feldspar, grey glassy quartz, black mica and hornblende — so its colour is a speckled average. Sandstones and limestones are coloured mostly by small amounts of iron oxides and hydroxides coating or cementing their grains, from cream and buff to red. Slate and dark stones owe their grey to black to fine carbon and dark minerals. The finish (polished, honed, flamed) and whether the stone is wet change how much of that mineral colour you see.
Relative to other materials in this section, not a measurement of a particular product.
| Process | What you see | Driven by | Slowing it |
|---|---|---|---|
| Soiling and crust formation | Streaky darkening; black gypsum crusts on sheltered limestone | Airborne pollution and how rain washes each surface | Gentle cleaning trials; good drip detailing |
| Iron staining | Rusty spots or tide marks | Oxidation of iron-bearing minerals or embedded fixings | Stainless fixings; specialist poultice for stains |
Colours that work with natural stone (granite, limestone, sandstone, slate) in a room
Granite is an igneous rock of quartz, alkali feldspar, plagioclase and mica, crystallised slowly enough for grains to be visible. The overall colour is set by the proportions: pink and red granites owe their colour to alkali feldspar; white and grey granites are richer in plagioclase and quartz; the black specks are biotite mica and hornblende. Commercial names are looser than geology — much of what is sold as 'black granite' is gabbro or other dark igneous rock with a different composition, and many 'granites' in kitchen showrooms are gneisses or other crystalline stones. Because granite is dense with very low porosity, polishing produces a deep mirror surface that shows each crystal's colour, and it changes little when wet.
Sedimentary stones are made of grains cemented together, and their colour usually comes from minor components rather than the main mineral. Quartz sand grains are colourless; a thin coating of iron hydroxide makes them buff or yellow, iron oxide makes them red, and a lack of iron leaves them white or grey. Limestones are mostly calcite and are cream to grey, with iron giving honey tones and organic matter grey or blue-grey. These stones are porous — pore volume can be as high as about a fifth — so they darken markedly when wet, absorb stains and pollution, and weather at the surface. A fresh-cut face can also change colour over the first months as iron minerals oxidise on exposure.
The same block gives very different surfaces. Polishing leaves a smooth face whose reflection is confined to the mirror direction, so crystal colours read saturated and the stone looks darker. Honing leaves a fine matt surface that scatters some white light and softens contrast. Flaming (thermal finishing) and bush-hammering shatter the surface into fine fragments that scatter strongly, so granite looks paler, greyer and more uniform; riven slate shows the natural cleavage with its subtle sheen. For approval, compare samples in the finish that will be supplied, and for external paving view them wet as well as dry, since that is how the stone will often be seen.
Outdoors, stone colour moves mostly through the surface. Pollution deposits unevenly, so discolouration follows how each face sheds water and appears in streaks. Acid rain dissolves calcium carbonate in limestone and marble, eroding exposed faces and forming black gypsum crusts in sheltered zones where the dissolved stone re-deposits with soot. Iron-bearing minerals and embedded iron fixings can rust, bleeding orange stains. Metals above stone leave their own marks: corrosion products washed from bronze and copper stain the masonry below green. Low-porosity granite and slate resist most of this for decades, which is why they are chosen where a stable colour matters.
Why: Natural variation across a block, or a sample from a different block.
Fix: Approve the actual slabs, and lay them out before cutting to plan how veining and colour fall.
Why: Oxidation of iron minerals in the stone or rusting of fixings or bedding contaminants.
Fix: Use stainless fixings and clean bedding; specialist poultices can reduce existing stains.
Why: Gypsum crusts with trapped soot in areas sheltered from rain-washing.
Fix: Gentle cleaning trials by a specialist; avoid abrasive or acid cleaners on calcareous stone.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
Granite consists mainly of quartz, alkali feldspar, plagioclase and mica; its pink and red tones come from alkali feldspar and its dark specks from biotite and hornblende.
Source: Granite
FactModerate evidence
Stone sold commercially as 'black granite' is usually gabbro, which has a different composition from granite.
Source: Granite
FactStrong evidence
The porosity of sedimentary stones such as limestone and sandstone can be as high as 20%, while granite's pore volume is very low, making porous stones more vulnerable to moisture-driven weathering.
Source: Preservation Brief 48: Preserving Grave Markers in Historic Cemeteries
FactStrong evidence
Acid rain erodes stones containing calcium carbonate such as limestone and marble, and airborne pollutants cause uneven, streaky discolouration that follows how each surface sheds water.
Source: Preservation Brief 48: Preserving Grave Markers in Historic Cemeteries
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.