Description
Genuine Opal – Gunheath China Clay Pit, Stenalees, Treverbyn, Cornwall
This natural Opal specimen from Gunheath China Clay Pit, Stenalees, Treverbyn, Cornwall, England, UK, is a genuine and carefully selected British mineral. The photograph provided shows the exact specimen you will receive. Please refer to the images for full sizing and scale details.
This crystal includes a fully hand-signed Premium Certificate of Authenticity confirming its genuine origin and documented Cornish provenance.
Mineral Type & Scientific Classification
Opal is a hydrated amorphous form of silica with the chemical formula SiO₂·nH₂O. Unlike quartz, opal does not possess a regular crystalline lattice and is therefore classified as a mineraloid rather than a true crystalline mineral.
Mineral Name: Opal
Chemical Formula: SiO₂·nH₂O
Mineral Group: Silica mineraloid
Crystal System: Amorphous
Hardness: 5.5–6.5 on the Mohs scale
Lustre: Vitreous to waxy
Transparency: Opaque to translucent
Cornish opal from china clay districts is generally common opal. It may exhibit milky white, cream, pale blue or soft grey tones depending on water content, trace elements and formation conditions.
Geological Setting – St Austell Granite & Kaolinisation
Gunheath China Clay Pit lies within the St Austell Granite, part of the Cornubian Batholith formed during late Carboniferous to early Permian tectonic events. This granite body underwent extensive hydrothermal alteration, resulting in world-renowned china clay (kaolin) deposits.
During kaolinisation, feldspar-rich granite altered chemically under the influence of mineral-rich fluids. As feldspar broke down into kaolinite, silica was released into solution. That silica later reprecipitated in fractures, joints and cavities as secondary silica minerals, including opal.
The geological processes involved include:
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Granite intrusion and cooling fracture development
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Hydrothermal fluid circulation
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Chemical alteration of feldspar to kaolinite
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Secondary silica deposition in open spaces
Associated minerals in the Gunheath area commonly 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 environment makes Cornish opal directly linked to the historic china clay industry of Treverbyn and St Austell.
Physical Characteristics & Natural Features
This specimen may display:
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Massive or botryoidal silica growth
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Smooth, waxy to vitreous surface
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Subtle translucency at thinner edges
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Natural granite or clay matrix attachment
Because opal is amorphous, it forms as nodular masses, vein fillings or layered deposits rather than distinct crystal faces. Surface texture and internal structure reflect the slow precipitation of silica-rich fluids within granite fractures.
Minor inclusions, matrix remnants or natural surface variation are authentic features formed during geological development and contribute to the uniqueness of each specimen.
Collectibility & Cornish Provenance
Cornwall is internationally recognised for its complex mineralogy and mining history. Opal from named china clay pits such as Gunheath is comparatively uncommon and desirable among collectors of British minerals.
Collectors value this material for:
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Documented St Austell Granite provenance
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Association with historic china clay workings
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Secondary silica mineralisation
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Recognised Cornwall locality
Specimens from Gunheath China Clay Pit represent a tangible piece of Cornwall’s geological and industrial heritage.
Authenticity & Presentation
This Opal specimen has been carefully chosen for its natural character and 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, England, UK.
A distinctive example of Cornish silica mineralisation, this piece combines geological history, regional provenance and natural mineral character in one authentic British specimen.
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