Rock
Arkose
Arkoz
The feldspar-rich, pink-to-red sandstone: the rock of Uluru and Colorado's red rocks.
Also known as: Arkosic sandstone
- Popularly: Natural stone (building and decor)
_1.jpg){kind=link}
In short
Arkose is a type of sandstone that is at least 25% feldspar. It forms when sand eroded from feldspar-rich rocks, usually granite, is deposited quickly, before the feldspar can weather into clay. That is why its grains are angular and vary in size. Its colour ranges from grey to reddish; the pink and red tones come from potassium feldspar. Australia's iconic Uluru and Colorado's Red Rocks and Garden of the Gods are made of arkose.
Photos
_1.jpg){kind=link}
How does it form?
Arkose usually comes from feldspar-rich igneous or metamorphic rocks such as granite. When these rocks erode, they release quartz and feldspar grains. Feldspar normally weathers chemically into clay; arkose forms when the sediment is deposited quickly, before this can happen, or when chemical weathering stays weak in a cold or dry climate. It therefore tends to build up in tectonically active settings, during the late stage and immediate aftermath of mountain building, near granite country. Because the grains do not travel far, they stay angular.
Uluru’s arkose formed from sediment laid down in the Amadeus Basin about 550 million years ago. Colorado’s Fountain Formation built up 290–340 million years ago from material eroded off the granites and gneisses of the Ancestral Rocky Mountains; the red of the rocks reflects that source.
What is it used for?
- A clue to past climates: Unweathered feldspar shows that sediment was laid down quickly or that the climate was cold or dry; geologists use arkose to trace ancient mountains and climates.
- Natural heritage and tourism: Famous rocks such as Uluru, Garden of the Gods and the Flatirons are made of arkose or arkose-bearing rocks.
- A natural concert venue: Colorado’s Red Rocks Amphitheatre is an open-air concert venue set among these red rocks.
What determines its value?
Arkose is a common sandstone of limited commercial value. Its real value lies in landscape and science: it forms famous landforms such as Uluru and Colorado's red rocks, and it tells why feldspar did not weather, which says something about climate and mountain building. Collectors prefer freshly broken specimens with clear pink feldspar grains.
This information is educational, not buying, selling or investment advice. Value varies enormously with quality and treatment.
How to recognise it
- 1
A pink, red, orangish or grey sandstone; up close, pink or white feldspar grains can be picked out between the quartz grains.
- 2
The grains are angular and vary in size; it does not look like mature sandstones made of clean, rounded sand grains.
- 3
It rarely contains fossils.
- 4
It is usually found near granite country, together with conglomerate.
Fakes and imitations
What shops usually don’t tell you:
- From a distance it can look like pink granite; arkose, however, is a sedimentary rock whose grains are cemented together, while granite's crystals have grown into each other.
- Greywacke is also a “dirty” sandstone; what sets greywacke apart is a dark clay matrix of more than 15% of its volume, while arkose is defined by at least 25% feldspar.
Myths and misconceptions
MythUluru is a giant boulder sitting on top of the ground.
FactUluru and Kata Tjuta are only the exposed tips of huge rock slabs that continue underground for up to 6 km; they are not single boulders resting on the ground.
Did you know?
Uluru's surface is actually flaky red with grey patches: the red is the rusting of iron in the arkose, and the grey is the rock's original colour.
Uluru and its neighbour Kata Tjuta are only the tips of huge rock slabs that continue underground for up to 6 km; Kata Tjuta, however, is conglomerate rather than arkose.
Colorado's Red Rocks Amphitheatre is a natural concert venue among the rocks of the arkose-bearing Fountain Formation; its 9,525-capacity seating area opened in 1941.
Where is it found?
Loading map…
- Uluru, Northern Territory, Australia · Major depositMade of late Neoproterozoic to Cambrian arkose deposited in the Amadeus Basin; its red colour comes from rusting iron, while fresh rock is grey.
- Red Rocks Amphitheatre, Morrison, Colorado, USA · Known localityA natural amphitheatre among the red rocks of the 290–340-million-year-old Fountain Formation, made mostly of conglomerate, sandstone or arkose.
- Garden of the Gods, Colorado Springs, Colorado, USA · Known localityRed rocks of the Fountain Formation; their reddish colour reflects the make-up of the granites and gneisses they were eroded from.
- Flatirons, Boulder, Colorado, USA · Known localityTilted, flat-faced slabs of the Fountain Formation.
Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →
Frequently asked questions
What is arkose?
Arkose is a type of sandstone that is at least 25% feldspar. It forms when sand eroded from feldspar-rich rocks, usually granite, is deposited quickly, before the feldspar can weather into clay. That is why its grains are angular and vary in size. Its colour ranges from grey to reddish; the pink and red tones come from potassium feldspar. Australia's iconic Uluru and Colorado's Red Rocks and Garden of the Gods are made of arkose.
Why is Uluru red?
Uluru is made of arkose. According to Parks Australia, the red of its surface comes from the rusting of iron found naturally in arkose; the rock's original colour is grey. As water and oxygen break down minerals in the rock, red flakes are left on the surface.
What is arkose used for?
A clue to past climates: Unweathered feldspar shows that sediment was laid down quickly or that the climate was cold or dry; geologists use arkose to trace ancient mountains and climates. Natural heritage and tourism: Famous rocks such as Uluru, Garden of the Gods and the Flatirons are made of arkose or arkose-bearing rocks. A natural concert venue: Colorado's Red Rocks Amphitheatre is an open-air concert venue set among these red rocks.
How hard is arkose?
It has a hardness of 6–7 on the Mohs scale. It can scratch glass; a steel knife scratches it only with difficulty. Its specific gravity is 2.2–2.8 g/cm³.
Where is arkose found?
Its best-known localities: Uluru, Northern Territory, Australia; Red Rocks Amphitheatre, Morrison, Colorado, USA; Garden of the Gods, Colorado Springs, Colorado, USA; Flatirons, Boulder, Colorado, USA.
How can you tell if arkose is real?
A pink, red, orangish or grey sandstone; up close, pink or white feldspar grains can be picked out between the quartz grains. The grains are angular and vary in size; it does not look like mature sandstones made of clean, rounded sand grains. It rarely contains fossils. From a distance it can look like pink granite; arkose, however, is a sedimentary rock whose grains are cemented together, while granite's crystals have grown into each other.
Is arkose valuable?
On the “street to diamond” value ladder it sits on step 2 of nine: Raw material by the tonne. Arkose is a common sandstone of limited commercial value. Its real value lies in landscape and science: it forms famous landforms such as Uluru and Colorado's red rocks, and it tells why feldspar did not weather, which says something about climate and mountain building. Collectors prefer freshly broken specimens with clear pink feldspar grains.
Sources
- Arkose · Wikipedia
- Geology — Uluṟu-Kata Tjuṯa National Park · Parks Australia
- Fountain Formation · Wikipedia
- Red Rocks Amphitheatre · Wikipedia
- Density of Solids · The Engineering ToolBox
External links
ReviewedLast checked: 4 October 2026
On the same step
Actinolite
A green amphibole that grows like rays: a raw material of nephrite jade and cat's-eye stones; its fibrous form is asbestos.
Amphibolite
A dark, heavy, salt-and-pepper metamorphic rock: basalt or gabbro transformed at depth; the stone of Neolithic axes.
Anatase
Rutile's sibling in titanium dioxide: its sharp, stretched double pyramids turn into rutile when heated.