Mineral
Proustite
Ag₃AsS₃Prustit
The silver ore that glows ruby red: but left in the light it darkens to black.
Also known as: Ruby silver · Light red silver ore · Ruby blende
- Popularly: Ore and raw material
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In short
Proustite is a mineral made of silver, arsenic and sulfur (Ag₃AsS₃). Because of its scarlet-vermilion, adamantine, transparent crystals it is also called “ruby silver” or “light red silver ore”. Together with acanthite and pyrargyrite, it is one of the few silver minerals that can contain enough silver to be mined. It is sensitive to light: crystals left in the light soon turn dull black. It is named after the French chemist Joseph Louis Proust, whose chemical analyses distinguished it in 1804 from pyrargyrite, its antimony counterpart.
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How does it form?
Proustite forms late in ore deposits made by hot water solutions (hydrothermal fluids), after the other minerals. It is found especially near the surface of deposits, in the oxidised and enriched zones where silver is deposited again. It occurs with silver and sulfide minerals such as native silver, native arsenic, acanthite, tetrahedrite and chlorargyrite. Its composition can grade into that of pyrargyrite, its antimony counterpart.
What is it used for?
- Silver ore: Together with acanthite and pyrargyrite, it is one of the few silver minerals that can contain enough silver to be mined.
- Collecting: Its vivid red crystals make it much sought after by collectors; it is stored away from light.
- History of chemistry: Proust’s analyses in 1804 showed that proustite and pyrargyrite are different minerals.
What determines its value?
Proustite can be an important source of silver, although according to the Handbook of Mineralogy it occurs at many localities but rarely as an important ore mineral. Its value to collectors is very high: vividly red, lustrous, sharp and undarkened crystals, especially from famous localities such as Chañarcillo, are much sought after. Because it darkens in light, well-kept specimens are prized.
This information is educational, not buying, selling or investment advice. Value varies enormously with quality and treatment.
How to recognise it
- 1
Scarlet or vermilion; adamantine lustre, fresh crystals transparent to translucent.
- 2
Its streak (the powder it leaves on unglazed porcelain) is vermilion.
- 3
Very soft (Mohs 2–2.5) and heavy (specific gravity about 5.6).
- 4
Left in the light, its surface darkens and turns dull, opaque black.
- 5
Usually found with native silver, acanthite and other silver minerals.
Fakes and imitations
What shops usually don’t tell you:
- Pyrargyrite (“dark ruby silver”) is very similar; it contains antimony instead of arsenic and is a darker red. The easiest distinction is the streak: proustite leaves a scarlet streak, pyrargyrite a purplish-red one.
- Cinnabar is also red and leaves a red streak; cinnabar, however, is a mercury mineral, not a silver one. A certain identification takes analysis.
Safety
Proustite contains arsenic. According to the World Health Organization, inorganic arsenic causes cancer in humans, and long-term exposure leads to serious health problems. Do not put proustite in your mouth, grind, sand or heat it, breathe its dust or put it in drinking water. Wash your hands after handling it and keep specimens out of reach of children.
Myths and misconceptions
MythProustite is a precious stone that, like ruby, gets its red colour from chromium.
FactProustite is a silver arsenic sulfide and has nothing to do with chromium. It is very soft (Mohs 2–2.5), darkens in light and contains arsenic; it is a collector's mineral, not a jewellery stone. The name “ruby silver” comes only from its colour.
Did you know?
Proustite crystals are transparent and very brilliant, but in the light they soon become dull black and opaque. That is why collections keep it in the dark.
The Chañarcillo mine in Chile has produced magnificent groups of large proustite crystals.
A proustite crystal can grow up to 8 cm long.
Where is it found?
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- Chañarcillo, near Copiapó, Atacama, Chile · Major depositYielded exceptional crystals and magnificent groups of large crystals (coordinate is Copiapó; the mine lies south of the town)
- Himmelsfürst mine, Brand-Erbisdorf (near Freiberg), Saxony, Germany · Historic sourceA Saxon silver mine cited by the Handbook of Mineralogy for proustite
- Jáchymov (Joachimsthal), Czechia · Historic sourceFamous old silver mines; proustite occurs here too
- Sainte-Marie-aux-Mines, Haut-Rhin, France · Known localityOld silver mines in the Vosges mountains
- Batopilas, Chihuahua, Mexico · Known localityA Mexican silver district cited by the Handbook of Mineralogy for proustite
Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →
Frequently asked questions
What is proustite?
Proustite is a mineral made of silver, arsenic and sulfur (Ag₃AsS₃). Because of its scarlet-vermilion, adamantine, transparent crystals it is also called “ruby silver” or “light red silver ore”. Together with acanthite and pyrargyrite, it is one of the few silver minerals that can contain enough silver to be mined. It is sensitive to light: crystals left in the light soon turn dull black. It is named after the French chemist Joseph Louis Proust, whose chemical analyses distinguished it in 1804 from pyrargyrite, its antimony counterpart.
Why is proustite kept in the dark?
Proustite is sensitive to light. Fresh crystals are transparent and bright red, but in the light they soon become dull black and opaque. That is why museums and collectors keep proustite in dark cabinets or light-proof boxes.
What is proustite used for?
Silver ore: Together with acanthite and pyrargyrite, it is one of the few silver minerals that can contain enough silver to be mined. Collecting: Its vivid red crystals make it much sought after by collectors; it is stored away from light. History of chemistry: Proust's analyses in 1804 showed that proustite and pyrargyrite are different minerals.
How hard is proustite?
It has a hardness of 2–2.5 on the Mohs scale. It is soft enough to scratch with a fingernail. Its specific gravity is 5.57–5.63 g/cm³.
Where is proustite found?
Its best-known localities: Chañarcillo, near Copiapó, Atacama, Chile; Himmelsfürst mine, Brand-Erbisdorf (near Freiberg), Saxony, Germany; Jáchymov (Joachimsthal), Czechia; Sainte-Marie-aux-Mines, Haut-Rhin, France; Batopilas, Chihuahua, Mexico.
How can you tell if proustite is real?
Scarlet or vermilion; adamantine lustre, fresh crystals transparent to translucent. Its streak (the powder it leaves on unglazed porcelain) is vermilion. Very soft (Mohs 2–2.5) and heavy (specific gravity about 5.6). Pyrargyrite (“dark ruby silver”) is very similar; it contains antimony instead of arsenic and is a darker red. The easiest distinction is the streak: proustite leaves a scarlet streak, pyrargyrite a purplish-red one.
Is proustite valuable?
On the “street to diamond” value ladder it sits on step 4 of nine: Fine dimension stone and ore. Proustite can be an important source of silver, although according to the Handbook of Mineralogy it occurs at many localities but rarely as an important ore mineral. Its value to collectors is very high: vividly red, lustrous, sharp and undarkened crystals, especially from famous localities such as Chañarcillo, are much sought after. Because it darkens in light, well-kept specimens are prized.
Is proustite dangerous?
Proustite contains arsenic. According to the World Health Organization, inorganic arsenic causes cancer in humans, and long-term exposure leads to serious health problems. Do not put proustite in your mouth, grind, sand or heat it, breathe its dust or put it in drinking water. Wash your hands after handling it and keep specimens out of reach of children.
Sources
- Proustite · Wikipedia
- Handbook of Mineralogy: Proustite · Mineralogical Society of America
- Dünyada ve Türkiye'de Gümüş (Maden Serisi) · MTA Genel Müdürlüğü
- Pyrargyrite · Wikipedia
- Arsenic (fact sheet) · World Health Organization
External links
ReviewedLast checked: 4 October 2026
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