# Proustite - Page: https://taslar.bumba.tr/en/stones/proustite/ - Turkish: Prustit - Also known as: Ruby silver, Light red silver ore, Ruby blende - Kind: Mineral - Classification: Minerals › Sulfides and sulfosalts - Chemical formula: Ag₃AsS₃ - Crystal system: Trigonal - Hardness (Mohs): 2–2.5 - Specific gravity: 5.57–5.63 g/cm³ - Lustre: Adamantine - Transparency: Transparent, Translucent - Colours: scarlet-vermilion, dull black after exposure to light - Streak: Vermilion - Cleavage: Distinct on {1011} - Fracture: Conchoidal to uneven; brittle - Value ladder: 4/9 step, Fine dimension stone and ore - Last checked: 2026-10-04 The silver ore that glows ruby red: but left in the light it darkens to black. 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. ## 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. ## How to recognise it - 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). - Left in the light, its surface darkens and turns dull, opaque black. - Usually found with native silver, acanthite and other silver minerals. ## Fakes and imitations - 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. ## 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. ## Myths and misconceptions - Myth: Proustite is a precious stone that, like ruby, gets its red colour from chromium. Fact: Proustite 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. ## Curious questions **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. ## 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. ## 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? - Chañarcillo, near Copiapó, Atacama, Chile (Major deposit): Yielded 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 source): A Saxon silver mine cited by the Handbook of Mineralogy for proustite - Jáchymov (Joachimsthal), Czechia (Historic source): Famous old silver mines; proustite occurs here too - Sainte-Marie-aux-Mines, Haut-Rhin, France (Known locality): Old silver mines in the Vosges mountains - Batopilas, Chihuahua, Mexico (Known locality): A Mexican silver district cited by the Handbook of Mineralogy for proustite ## Sources - [Proustite](https://en.wikipedia.org/wiki/Proustite), Wikipedia - [Handbook of Mineralogy: Proustite](https://www.handbookofmineralogy.org/pdfs/proustite.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 - [Pyrargyrite](https://en.wikipedia.org/wiki/Pyrargyrite), Wikipedia - [Arsenic (fact sheet)](https://www.who.int/news-room/fact-sheets/detail/arsenic), World Health Organization Source: https://taslar.bumba.tr/en/stones/proustite/ · Licence: CC BY-SA 4.0