How do mineral specimens form?
41 of the 109 species here are described as secondary or as forming in the oxidised zone. Most display material grew from a rock breaking down, not with it.
Mostly by decay, not by growth with the rock. Of the 109 species sheets in this site's reference set, 41 describe the species as secondary or as forming in the oxidised zone of an orebody — 25 use the phrase oxidised zone explicitly. A crystal needs an open space to grow into, and a decaying orebody is what makes the spaces.
In short
- 41 of 109 species here formed from something else breaking down. Counted from the Occurrence field of every species sheet in this site's reference set: 25 say oxidised zone, 33 say secondary, and the union of the two is 41.
- Crystals need a void, and that is the real constraint. A mineral growing in solid rock competes for space and ends up as an interlocking grain. The same mineral growing into an open cavity makes faces. Almost every specimen worth displaying grew into a hole.
- The oxidised zone is where an orebody meets the weather. Groundwater carrying oxygen works down through a sulphide orebody, dissolves it, and reprecipitates the metals as carbonates, sulphates, arsenates, phosphates and vanadates — the brightly coloured species that fill collections.
- That is why the famous localities are famous. Tsumeb, Ojuela, Broken Hill, Bou Azzer and the Cornish copper mines are not unusually rich in species; they are unusually deep oxidised zones with unusual chemistry, opened by mining.
- It also explains why the supply is finite. An oxidised zone is a single geological event with a fixed volume, and once a mine has worked through it there is no more. The consequence for collectors is on what ex mine means on a specimen label.
| Route | What happens | What the specimen looks like | Species on this site |
|---|---|---|---|
| Supergene, in the oxidised zone | Oxygenated groundwater dissolves a sulphide orebody and reprecipitates the metals | Bright colours, small sharp crystals, crusts and botryoidal masses on gossan | Azurite, malachite, mimetite, pyromorphite, olivenite, cerussite — 41 of 109 |
| Hydrothermal, in a vein | Hot mineralising fluid deposits directly into an open fissure | Large well-formed crystals on a vein wall, often in a clear paragenetic sequence | Fluorite, baryte, galena, quartz, cassiterite |
| Cavity filling in igneous rock | Late-stage or groundwater solutions fill gas bubbles and fractures in lava | Zeolites and silica lining a rounded void | Analcime, heulandite, stilbite, prehnite |
| Metamorphic and pegmatitic growth | Crystals grow during recrystallisation, or from a volatile-rich late magmatic melt | Large crystals in a solid rock, often without free faces unless a pocket existed | Vesuvianite, epidote, kyanite, muscovite, tourmaline |
The count, and what it says about what collectors actually own
Method. On 20 September 2026 the Handbook of Mineralogy sheet for every species this site covers — 109 sheets after removing duplicates, mirror copies and non-species files — was converted to text and the Occurrence field extracted and searched.
25 sheets use the phrase oxidised zone or oxidation zone. 33 describe the species as secondary. The union is 41 of 109 — 38%.
That number is a floor rather than a ceiling, because the Occurrence field is terse and some secondary species describe their origin without either word. But even as a floor it changes the shape of the answer to the question. More than a third of the species a collector handles did not form when the rock formed. They formed later, out of that rock coming apart.
This is the collector's version of a point a general geology source makes differently. The British Geological Survey introduction to rocks and minerals sets out the standard three-way division — igneous, sedimentary and metamorphic — which is the right framework for rocks. For specimens it is the wrong emphasis, because the great majority of cabinet minerals are not representative of any of the three; they are the contents of holes in them.
Why crystals need a hole, and what that implies for a specimen
A mineral crystallising in a closed system, surrounded on all sides by other crystallising minerals, cannot develop faces. It fills the space left to it and ends up as an anhedral grain with an irregular boundary. A granite is full of quartz and none of it looks like a quartz crystal.
Faces require unimpeded growth, and unimpeded growth requires a void. So a display specimen is, almost by definition, evidence that a hole existed: a vug in a vein, a gas bubble in a lava, a solution cavity in a limestone, an open fracture, or the space left behind when a soluble mineral dissolved away.
Three consequences follow, and all three are practical.
Crystal size is limited by the void, not by time. A pocket 3 cm across produces crystals under 3 cm however long the process ran. That is why large crystals come from settings with large openings — pegmatite pockets, deep solution cavities, big vein vugs — rather than from settings that happened to be slow.
The matrix is the evidence. A specimen still attached to its vug wall carries its own account of how it formed and where it sat. Trimming that away for a cleaner presentation removes information, which is the argument made on specimen sizes and grading.
Paragenesis is readable. When several species occupy one vug, the sequence in which they grew is usually visible: the earliest sit directly on the wall, the latest sit on top of everything else. Working that out is one of the genuine pleasures of a good cabinet piece and is a large part of what separates a specimen from a rock — see how to identify a mineral specimen.
The oxidised zone, in the order it happens
This is the process behind 41 of the 109 species here, and it is worth following through once.
1. A sulphide orebody sits below the water table, stable because the water there carries no free oxygen. Galena, sphalerite, chalcopyrite, pyrite and arsenopyrite are all comfortable.
2. Erosion lowers the land surface, or the water table falls. The top of the orebody now sits in oxygenated groundwater, and every sulphide in it becomes unstable at once.
3. Sulphides oxidise, and the products are acidic. Pyrite oxidation produces sulphuric acid — the same reaction that destroys pyrite specimens in a damp drawer, described on why is my pyrite crumbling. The acid attacks the rest of the orebody and the wall rock.
4. Metals go into solution and travel a short distance. Copper, lead, zinc, arsenic and vanadium move with the water until something changes — usually a carbonate wall rock neutralising the acid.
5. They reprecipitate as new species, in the voids the dissolution has just created. Lead meets carbonate and becomes cerussite; copper meets carbonate and becomes malachite and azurite; lead meets arsenate and becomes mimetite. The dissolution makes the space and the reprecipitation fills it, which is why oxidised zones are so productive of crystallised material.
6. A gossan is left at the top — the iron-stained residue of an orebody that has had its metals removed. Prospectors follow gossans down; collectors follow them down too, for the same reason.
Every locality this site covers in depth is a variation on that sequence: Tsumeb exceptionally deep, Ojuela exceptionally arsenate-rich, the Caldbeck Fells exceptionally varied for their size, Cornwall exceptionally long-worked.
What this means for buying and for labels
If the specimen is the contents of a hole in an orebody, then the hole's identity is part of the specimen. That is not sentiment; it is the reason locality data carries the weight it does in this hobby.
Species and locality together are a testable claim. An oxidised-zone assemblage is characteristic: certain species occur together because the same chemistry produced them. A combination that does not belong together is a reason to ask questions, which is the argument on what to look for when buying minerals online.
Supply is genuinely finite in a way that is not true of most collectables. An oxidised zone has a volume; when the mine has worked through it, the material in circulation is all there will be. That is why old collection material is where the classic localities now live, and why a locality that closed sixty years ago still supplies the market.
We hold no stock and quote no prices — there is no catalogue on this site. If you are looking for material from a specific working, the route is the wanted list. The rest of this section is indexed at collecting guides, and the species covered here are at mineral species.