Pyrargyrite
Pyrargyrite is Ag3SbS3, hardness 2.5, density 5.82, deep red with a purplish red streak. It darkens irreversibly on exposure to light and must be stored dark.
Pyrargyrite is a silver antimony sulphosalt, Ag3SbS3, hexagonal, hardness 2.5 and density 5.82, with an adamantine lustre, a deep red crystal colour and a purplish red streak. It is 59.76% silver by mass. The Handbook records it as translucent and darkening with exposure to light, so a display specimen loses its colour permanently unless it is kept in the dark.
In short
- Light destroys the colour, and the damage is not reversible. The Handbook records pyrargyrite as translucent and darkening on exposure to light. A deep red crystal on an open shelf becomes an opaque grey-black one, and no cleaning brings it back. Store it in a closed drawer.
- Four species in this site's 158-sheet reference set have a refractive index above 3.0, and three of the four are arsenic or antimony sulphides. Hematite 3.15 to 3.22 but opaque, proustite 3.0877, pyrargyrite 3.084 and orpiment 3.02. That is why the ruby silvers look internally lit rather than merely red.
- The streak separates it from every red mineral it resembles. Pyrargyrite streaks purplish red, proustite vermilion, pyrostilpnite yellow-orange and cuprite brownish red. Streak is the one test that survives a darkened surface, and the method is on our streak plate note.
- It is 59.76% silver by mass, computed from Ag3SbS3, which matches the Handbook's own analysis of Freiberg material at 60.17% and Săcărîmb material at 59.82%. It is a genuine silver ore, not a curiosity.
- Dimorphous with pyrostilpnite. Same formula, Ag3SbS3, different structure: pyrostilpnite is hyacinth-red with a yellow-orange streak and density 5.94. Two species, one composition, and the streak tells them apart.
| Species | Formula | Hardness | Density | Streak | The separating feature |
|---|---|---|---|---|---|
| Pyrargyrite | Ag3SbS3 | 2.5 | 5.82 | Purplish red | Deep red, adamantine; darkens in light |
| Proustite | Ag3AsS3 | 2 to 2.5 | 5.57 | Vermilion | Scarlet-vermilion rather than deep red; the arsenic analogue, and it darkens too |
| Pyrostilpnite | Ag3SbS3 | 2 | 5.94 | Yellow-orange | Same formula as pyrargyrite; hyacinth-red, lemon-yellow in transmitted light |
| Cuprite | Cu2O | 3.5 to 4 | 6.14 | Brownish red | Harder, denser, and a copper oxide rather than a silver sulphosalt |
| Crocoite | PbCrO4 | 2.5 to 3 | 6.0 to 6.1 | Orange-yellow | Orange rather than red; sectile; prismatic habit |
| Realgar | AsS | 1.5 to 2 | 3.56 | Red-orange | Far lighter, softer, and it disintegrates to powder in light rather than merely darkening |
Why the ruby silvers look the way they do
Pyrargyrite's refractive index is the reason. The Handbook of Mineralogy gives omega = 3.084 measured with lithium light. Across the 158 species sheets in this site's reference set, only four exceed 3.0, and three of those four are arsenic or antimony sulphides: proustite at 3.0877, pyrargyrite at 3.084 and orpiment at 3.02. The fourth, hematite at 3.15 to 3.22, is opaque, so it never shows what a figure like that does to light.
For comparison, the next species down after orpiment is realgar at 2.704, then crocoite at 2.66. Diamond, which is not in the set, is 2.42. A translucent crystal with a refractive index over 3 does something to light that almost nothing else a collector handles does, and that internal fire is what the word ruby in the old dealer's name is pointing at.
It also explains the lustre the Handbook records: adamantine, not vitreous. High refractive index and adamantine lustre are the same fact described twice.
Light damage, and what to do about it
The Handbook's optical entry opens translucent, darkens with exposure to light. Only three species in the reference set carry that wording: pyrargyrite, proustite and marcasite. It is a statement about a permanent change, not about a temporary appearance.
The behaviour belongs to a wider group of light-sensitive minerals. The NatSCA guidance on the care and conservation of geological specimens puts it plainly: light will irreversibly damage some mineral species, and susceptible species should be stored in light-proof containers. It names the arsenic sulphides as the most susceptible and adds amethyst and green fluorite as faders.
The practical rule for a ruby silver is that it is a drawer specimen, not a cabinet specimen. That is an unwelcome conclusion for a mineral whose whole appeal is visual, and it is the honest one. If it must be displayed, display it briefly, under low light, and accept that you are spending the colour. Our notes on which minerals fade in light and lighting a mineral display cabinet cover the general case.
Localities, and the company it keeps
The Handbook describes pyrargyrite as an important ore of silver, not uncommon in oxidised silver deposits but rarely in fine specimens. It forms in hydrothermal veins as a primary late-stage, low-temperature mineral and also by secondary processes, associated with native silver, acanthite, tetrahedrite, other silver sulphosalts, calcite, dolomite and quartz.
The named localities are the historic silver districts: St Andreasberg in the Harz and the Himmelsfürst mine near Freiberg in Saxony; Hiendelaencina in Spain; Jáchymov and Příbram; the Comstock Lode in Nevada; Cobalt in Ontario; Colquechaca in Bolivia; and many Mexican districts, especially Fresnillo.
A specimen with a secure old label from one of those districts is worth more than a better crystal without one, and the argument for that is on whether a specimen without a locality is worth buying. The rest of this section is indexed at the species index. There is no catalogue here and nothing is offered for sale; the wanted list is the only commercial route.