Mineral
Erythrite
Co₃(AsO₄)₂·8H₂OEritrit
The “cobalt bloom” that coats cobalt ores like a pink-red crust: an arsenic mineral that guided prospectors.
Also known as: Cobalt bloom
- Popularly: Common stone
In short
Erythrite is a mineral made of cobalt, arsenic and water (Co₃(AsO₄)₂·8H₂O). It is known for its colour, ranging from crimson to peach red and pink. Because it forms as a crust on cobalt arsenide minerals where they are exposed to air, it is called “cobalt bloom”. Prospectors used it as a sign that cobalt and native silver might be nearby. Its name comes from the Greek for “red”. It has nothing to do with erythritol, the sweetener.
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How does it form?
Erythrite is a secondary mineral: it forms in the oxidised zone of ore deposits containing cobalt, nickel and arsenic, the part near the surface exposed to air and water. As cobalt arsenide minerals such as cobaltite and skutterudite break down, the cobalt and arsenic released combine with water to form erythrite. That is why it is usually seen as a pink crust or powder on the surface of the ore minerals. It can mix in any proportion with its nickel counterpart, annabergite.
According to the Handbook of Mineralogy, erythrite occurs at many localities, but few of them are important. Fine specimens with large crystals come from Schneeberg, Wittichen and Richelsdorf in Germany and from the mines of the Bou Azzer district in Morocco; Jáchymov in Czechia, Cornwall in England, Cobalt in Ontario and the Blackbird mine in Idaho are among its other known localities.
What is it used for?
- Prospecting: The pink crust on cobalt ores tells prospectors that cobalt and native silver might be nearby.
- Collecting: Its well-formed deep red crystals are sought by collectors.
What determines its value?
Erythrite is valued not as an ore but as a sign of cobalt deposits and as a collector's mineral. Collectors look for large, lustrous, deep red crystals or star-shaped sprays; specimens from Bou Azzer in Morocco and Schneeberg in Germany are the best known. Powdery crusts have no collector value.
This information is educational, not buying, selling or investment advice. Value varies enormously with quality and treatment.
How to recognise it
- 1
Crimson, peach red, pale rose or pink; usually found as powdery crusts or radiating, star-shaped sprays of needles.
- 2
Very soft (Mohs 1.5–2.5); its thin flakes can be bent and it can be cut with a knife.
- 3
Its streak is pale red to pink.
- 4
It has a pearly lustre on cleavage surfaces.
- 5
Found with cobaltite, skutterudite and other cobalt–nickel–arsenic minerals.
Fakes and imitations
What shops usually don’t tell you:
- Erythrite forms a complete series with its nickel counterpart, annabergite, which is apple green. Specimens of intermediate composition can be paler in colour.
Safety
Erythrite contains arsenic. According to the World Health Organization, inorganic arsenic causes cancer in humans, and long-term exposure leads to serious health problems. Because erythrite usually occurs as powdery crusts, do not breathe its dust, put it in your mouth, grind or heat it, or put it in drinking water. Wash your hands after handling it and keep specimens out of reach of children.
Myths and misconceptions
MythBecause it is pink and soft, erythrite is a harmless ornamental stone.
FactErythrite contains arsenic and is very soft (Mohs 1.5–2.5). It is not used as an ornamental stone; it is a collector's mineral, and its dust should never be breathed in or swallowed.
Did you know?
Erythrite was first described in 1832 from the Daniel mine at Schneeberg, in Saxony, Germany.
Prospectors who saw the pink crust on cobalt ores knew that cobalt and native silver might be nearby.
Its crystals are rarely well formed but can grow up to 10 cm; the Bou Azzer district in Morocco is known for its large crystals.
Where is it found?
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- Schneeberg, Saxony, Germany · Type localityErythrite was first described in 1832 from the Daniel mine here; yields fine specimens with large crystals
- Jáchymov (Joachimsthal), Czechia · Known localityOld cobalt–silver mines cited by the Handbook of Mineralogy for erythrite
- Botallack mine, St Just, Cornwall, England · Known localityCornwall's famous old clifftop mine; erythrite occurs here
- Cobalt, Ontario, Canada · Known localityA town named for its cobalt and silver mines; erythrite occurs here
Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →
Frequently asked questions
What is erythrite?
Erythrite is a mineral made of cobalt, arsenic and water (Co₃(AsO₄)₂·8H₂O). It is known for its colour, ranging from crimson to peach red and pink. Because it forms as a crust on cobalt arsenide minerals where they are exposed to air, it is called “cobalt bloom”. Prospectors used it as a sign that cobalt and native silver might be nearby. Its name comes from the Greek for “red”. It has nothing to do with erythritol, the sweetener.
Are erythrite and the sweetener erythritol the same thing?
No. Erythrite is a pink-red mineral containing cobalt and arsenic, and it is toxic. Erythritol is a sweetener used in place of sugar. Their names are similar, but they have nothing to do with each other; never put erythrite in your mouth.
What is erythrite used for?
Prospecting: The pink crust on cobalt ores tells prospectors that cobalt and native silver might be nearby. Collecting: Its well-formed deep red crystals are sought by collectors.
How hard is erythrite?
It has a hardness of 1.5–2.5 on the Mohs scale. It is soft enough to scratch with a fingernail. Its specific gravity is 3.06–3.14 g/cm³.
Where is erythrite found?
Its best-known localities: Schneeberg, Saxony, Germany; Jáchymov (Joachimsthal), Czechia; Botallack mine, St Just, Cornwall, England; Cobalt, Ontario, Canada.
How can you tell if erythrite is real?
Crimson, peach red, pale rose or pink; usually found as powdery crusts or radiating, star-shaped sprays of needles. Very soft (Mohs 1.5–2.5); its thin flakes can be bent and it can be cut with a knife. Its streak is pale red to pink. Erythrite forms a complete series with its nickel counterpart, annabergite, which is apple green. Specimens of intermediate composition can be paler in colour.
Is erythrite valuable?
On the “street to diamond” value ladder it sits on step 3 of nine: Building and industrial stone. Erythrite is valued not as an ore but as a sign of cobalt deposits and as a collector's mineral. Collectors look for large, lustrous, deep red crystals or star-shaped sprays; specimens from Bou Azzer in Morocco and Schneeberg in Germany are the best known. Powdery crusts have no collector value.
Is erythrite dangerous?
Erythrite contains arsenic. According to the World Health Organization, inorganic arsenic causes cancer in humans, and long-term exposure leads to serious health problems. Because erythrite usually occurs as powdery crusts, do not breathe its dust, put it in your mouth, grind or heat it, or put it in drinking water. Wash your hands after handling it and keep specimens out of reach of children.
Sources
- Erythrite · Wikipedia
- Handbook of Mineralogy: Erythrite · Mineralogical Society of America
- Arsenic (fact sheet) · World Health Organization
External links
ReviewedLast checked: 4 October 2026
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