Description
Genuine Opal – Gunheath China Clay Pit, Stenalees, Treverbyn, Cornwall
This natural Opal specimen from the renowned Gunheath China Clay Pit, Stenalees, Treverbyn, Cornwall, UK, is a genuine British mineral carefully selected for quality, locality significance and display appeal. The photograph provided shows the exact specimen you will receive. Please refer to the images for full sizing and scale details.
Your crystal includes a fully hand-signed Premium Certificate of Authenticity confirming its genuine origin and Cornish provenance.
Mineral Classification & Composition
Opal is a hydrated amorphous form of silica with the chemical composition SiO₂·nH₂O. Unlike crystalline quartz, opal lacks a defined crystal lattice structure and is classified as a mineraloid rather than a true mineral.
Mineral Name: Opal
Chemical Composition: Hydrated Silica (SiO₂·nH₂O)
Mineral Group: Silica mineraloid
Crystal System: Amorphous (no true crystal structure)
Hardness: 5.5–6.5 on the Mohs scale
Lustre: Vitreous to waxy
Transparency: Opaque to translucent
Cornish opal from china clay pits is typically common opal rather than precious opal, although some specimens may show subtle internal colour variation. The material often appears milky white, cream, pale blue, or faintly translucent depending on silica deposition and water content.
Geological Setting – Cornish China Clay Deposits
Gunheath China Clay Pit lies within the St Austell Granite district, one of the most significant granite bodies in southwest England. The china clay (kaolin) deposits formed through the hydrothermal alteration of feldspar-rich granite.
As mineral-rich fluids circulated through fractures in the granite, feldspars altered to kaolinite. Silica released during this alteration process was remobilised and redeposited in cavities, fractures and voids, forming secondary silica minerals including opal.
The geological environment includes:
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Variscan granite intrusion
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Hydrothermal alteration zones
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Kaolinisation processes
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Secondary silica deposition
Associated minerals in the Gunheath area may include:
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Kaolinite
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Quartz
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Tourmaline
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Fluorite
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Mica
This geological setting gives Cornish opal a strong connection to the historic china clay industry of St Austell.
Physical Characteristics & Surface Features
This Opal specimen may display:
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Smooth botryoidal or massive surfaces
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Waxy to vitreous lustre
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Natural fracture patterns
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Subtle translucency at thinner edges
Because opal is amorphous, it does not form traditional crystal faces. Instead, it often develops rounded, layered or vein-like growth forms within host rock. Minor inclusions or matrix attachment are natural features resulting from formation within granite cavities.
Variations in colour and texture reflect changing silica concentration and water content during deposition.
Collectibility & Cornish Mining Heritage
Cornwall is internationally recognised for its complex mineralogy and long mining history. Opal from named china clay pits such as Gunheath is valued for its locality documentation and geological context.
Collectors appreciate Cornish opal for:
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Documented St Austell Granite provenance
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Association with historic china clay workings
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Distinctive silica formation within granite
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Strong regional mineral heritage
Specimens from Gunheath China Clay Pit provide a tangible link to one of Cornwall’s most important industrial mineral regions.
Authenticity & Presentation
This Opal specimen has been carefully chosen for its natural character and Cornish locality significance. The image shown is of the exact specimen you will receive.
Included with your purchase is a fully hand-signed Premium Certificate of Authenticity confirming this is a genuine Opal specimen from Gunheath China Clay Pit, Stenalees, Treverbyn, Cornwall, UK.
A distinctive example of Cornish silica mineralisation, this piece offers geological interest, regional provenance and display appeal in one authentic British mineral specimen.
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