# Erythrite

- Page: https://taslar.bumba.tr/en/stones/erythrite/
- Turkish: Eritrit
- Also known as: Cobalt bloom
- Kind: Mineral
- Classification: Minerals › Phosphates
- Chemical formula: Co₃(AsO₄)₂·8H₂O
- Crystal system: Monoclinic
- Hardness (Mohs): 1.5–2.5
- Specific gravity: 3.06–3.14 g/cm³
- Lustre: Adamantine, Pearly
- Transparency: Transparent, Translucent
- Colours: crimson, peach red, pale rose, pink
- Streak: Pale red to pink
- Cleavage: Perfect on {010}; poor on {100} and {102}
- Fracture: Sectile; thin laminae are flexible
- Value ladder: 3/9 step, Building and industrial stone
- Last checked: 2026-10-04

The “cobalt bloom” that coats cobalt ores like a pink-red crust: an arsenic mineral that guided prospectors.

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.

## 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.

## How to recognise it

- 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.
- It has a pearly lustre on cleavage surfaces.
- Found with cobaltite, skutterudite and other cobalt–nickel–arsenic minerals.

## Fakes and imitations

- Erythrite forms a complete series with its nickel counterpart, annabergite, which is apple green. Specimens of intermediate composition can be paler in colour.

## 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.

## Myths and misconceptions

- Myth: Because it is pink and soft, erythrite is a harmless ornamental stone. Fact: Erythrite 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.

## Curious questions

**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.

## 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.

## 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?

- Schneeberg, Saxony, Germany (Type locality): Erythrite was first described in 1832 from the Daniel mine here; yields fine specimens with large crystals
- Jáchymov (Joachimsthal), Czechia (Known locality): Old cobalt–silver mines cited by the Handbook of Mineralogy for erythrite
- Botallack mine, St Just, Cornwall, England (Known locality): Cornwall's famous old clifftop mine; erythrite occurs here
- Cobalt, Ontario, Canada (Known locality): A town named for its cobalt and silver mines; erythrite occurs here

## Sources

- [Erythrite](https://en.wikipedia.org/wiki/Erythrite), Wikipedia
- [Handbook of Mineralogy: Erythrite](https://www.handbookofmineralogy.org/pdfs/erythrite.pdf), Mineralogical Society of America
- [Arsenic (fact sheet)](https://www.who.int/news-room/fact-sheets/detail/arsenic), World Health Organization


Source: https://taslar.bumba.tr/en/stones/erythrite/ · Licence: CC BY-SA 4.0