# Acanthite

- Page: https://taslar.bumba.tr/en/stones/acanthite/
- Turkish: Akantit
- Also known as: Argentite (name of the high-temperature form, often used loosely), Silver glance
- Kind: Mineral
- Classification: Minerals › Sulfides and sulfosalts
- Chemical formula: Ag₂S
- Crystal system: Monoclinic
- Hardness (Mohs): 2–2.5
- Specific gravity: 7.2–7.24 g/cm³
- Lustre: Metallic
- Transparency: Opaque
- Colours: iron black
- Streak: Black
- Cleavage: Indistinct
- Fracture: Uneven; sectile
- Value ladder: 3/9 step, Building and industrial stone
- Last checked: 2026-10-04

Silver's black sulfide: one of the world's main silver ores and the mineral that coats tarnished native silver.

Acanthite is a mineral made of silver and sulfur (silver sulfide, Ag₂S). It is an iron-black mineral with a metallic lustre, soft enough to be cut with a knife. Acanthite, proustite and pyrargyrite are among the few silver minerals that can contain enough silver to be mined. Silver sulfide is stable as acanthite below 173 °C; its cubic form above that temperature is called argentite. Its name comes from the Greek for “thorn”, after the shape of its crystals.

## How does it form?

Acanthite is a common silver mineral in sulfide veins left by moderately low-temperature hot water solutions (hydrothermal fluids) moving through cracks. It also forms near the surface of deposits, in zones of secondary enrichment where silver is dissolved and deposited again. Above 173 °C silver sulfide crystallises as cubic argentite; on cooling it turns into acanthite but often keeps its cubic or octahedral outer shape. These relict crystals can grow up to 8 cm.

## What is it used for?

- **Silver ore:** It is one of the few silver minerals that can contain enough silver to be mined. Silver sulfide minerals are concentrated by flotation.
- **Science:** The change from argentite to acanthite is a good example of how the same compound can take different crystal structures at different temperatures.
- **Collecting:** Its sharp crystals and specimens found with native silver are sought by collectors.

## How to recognise it

- Iron black, metallic and completely opaque; its streak is black.
- Very soft (Mohs 2–2.5); a knife cuts it like lead (it is sectile).
- Very heavy (specific gravity about 7.2): nearly three times as heavy as a piece of quartz the same size.
- Its crystals are often cubes or octahedra; these are really argentite crystals formed at high temperature that turned into acanthite as they cooled but kept their outer shape.
- Usually found with native silver, proustite, pyrargyrite, galena, chalcopyrite, sphalerite, calcite and quartz.

## Fakes and imitations

- Galena is also a grey-black, heavy, metallic sulfide; galena, however, is brittle and breaks into steps along its perfect cubic cleavage, while acanthite has no distinct cleavage and can be cut with a knife like lead.

## What determines its value?

Acanthite is a valuable ore for its silver: about 87% of the weight of pure silver sulfide is silver. Most silver produced today is a by-product of lead–zinc, copper and gold mining, and silver minerals such as acanthite also occur within these ores. Collectors look for sharp, lustrous crystals, especially large cubes and octahedra that kept the shape of argentite, and specimens found with native silver.

This information is educational, not buying, selling or investment advice. Value varies enormously with quality and treatment.

## Myths and misconceptions

- Myth: Cube-shaped black silver sulfide crystals are argentite. Fact: Argentite is stable only above 173 °C. The cubes and octahedra we see at room temperature are crystals that turned into acanthite as they cooled but kept the outer shape of argentite; today they are really acanthite.

## Curious questions

**Why does silver tarnish?**

Silver reacts with sulfur compounds in the air and in sweat, forming silver sulfide on its surface. The same happens in nature: the black coating on many native silver specimens exposed to air or hot water is acanthite, the silver sulfide mineral.

## Did you know?

- The black coating on many native silver specimens exposed to air or hot water is acanthite.
- Acanthite is sensitive to light (photosensitive).
- It was first described in 1855 from the mines of Jáchymov (Joachimsthal), in today's Czechia. The “Joachimsthaler” silver coins minted there from 1520 became known in German as “Thaler”, the origin of the word “dollar”.

## Where is it found?

- Jáchymov (Joachimsthal), Karlovy Vary, Czechia (Type locality): Acanthite was first described in 1855 from specimens here; the “Joachimsthaler” silver coins minted from 1520 are the origin of the word “dollar”
- Freiberg, Saxony, Germany (Historic source): One of the old Saxon silver-mining districts the Handbook of Mineralogy cites for fine acanthite crystals
- Guanajuato, Mexico (Known locality): The Rayas and other mines; among the localities the Handbook of Mineralogy cites for acanthite
- Comstock Lode, Virginia City, Nevada, USA (Historic source): The famous US silver deposit; the Handbook of Mineralogy records acanthite here
- Cobalt, Ontario, Canada (Known locality): Acanthite occurs in various silver mines here

## Sources

- [Acanthite](https://en.wikipedia.org/wiki/Acanthite), Wikipedia
- [Handbook of Mineralogy: Acanthite](https://www.handbookofmineralogy.org/pdfs/acanthite.pdf), Mineralogical Society of America
- [Dünyada ve Türkiye'de Gümüş (Maden Serisi)](https://www.mta.gov.tr/v3.0/sayfalar/bilgi-merkezi/maden-serisi/dunyada-ve-turkiyede-gumus.pdf), MTA Genel Müdürlüğü
- [Jáchymov](https://en.wikipedia.org/wiki/J%C3%A1chymov), Wikipedia


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