Opal Gunheath China Clay Pit Cornwall UK Natural Cornish Opal Specimen

Genuine Opal – Gunheath China Clay Pit, Stenalees, Treverbyn, Cornwall, UK

This Opal specimen from Gunheath China Clay Pit, Stenalees, Treverbyn, Cornwall, England, is a genuine and carefully selected Cornish mineral example. 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 identity and documented Cornish locality.

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Mineral Species & Scientific Classification

Opal is a hydrated amorphous form of silica with the general chemical formula SiO₂·nH₂O. Unlike quartz, opal does not possess a defined crystal structure and is classified as a mineraloid rather than a true mineral.

Mineral Name: Opal
Chemical Composition: Hydrated Silicon Dioxide (SiO₂·nH₂O)
Mineral Group: Silica (Mineraloid)
Crystal System: Amorphous
Hardness: Approximately 5.5–6.5 on the Mohs scale
Lustre: Vitreous to resinous
Transparency: Opaque to translucent

Cornish opal from china clay districts is typically common opal rather than precious opal, though subtle internal colour variation and translucency can occur. The material may appear milky white, cream, pale blue, or softly tinted depending on trace elements and formation conditions.

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Geological Setting – St Austell Granite & China Clay Deposits

Gunheath China Clay Pit lies within the St Austell Granite, part of the Cornubian Batholith formed during late Carboniferous to early Permian magmatic activity. This granite body underwent intense hydrothermal alteration, leading to the formation of kaolin (china clay) deposits that have been mined for centuries.

Opal in this region forms through:

• Silica-rich hydrothermal fluids

• Weathering and alteration of feldspar within granite

• Deposition in fractures, cavities and clay-altered zones

As feldspar decomposed into kaolinite, excess silica was mobilised and redeposited as opaline material. This process often produces nodular, vein-like or cavity-filling opal associated with kaolinised granite, quartz and other secondary silica phases.

Cornish china clay pits are well known among collectors for producing interesting secondary silica minerals linked to this alteration environment.

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Physical Characteristics & Matrix Association

This specimen displays natural opal formation within its original host matrix, preserving the geological context of formation. Features may include:

• Smooth, waxy to vitreous surface texture

• Opaque to slightly translucent silica

• Cream, white or pale-toned coloration

• Natural fracture patterns typical of amorphous silica

• Association with kaolinised granite or clay matrix

Opal’s internal structure consists of microscopic silica spheres arranged irregularly. In common opal, these spheres lack the uniformity required to produce strong play-of-colour, resulting instead in a soft, even appearance.

The material is untreated and presented in its natural state.

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Collectibility & Cornish Provenance

Cornwall has a long and internationally recognised mining history. Specimens from documented china clay localities such as Gunheath are valued for their regional geological importance.

Collectors appreciate Cornish opal for:

• Clear St Austell Granite provenance

• Association with historic china clay workings

• Secondary silica mineralisation context

• Authentic British mining heritage

This piece makes an excellent addition to a UK mineral collection, a Cornish locality display, or an educational geology cabinet focused on granite alteration processes.

Opal has been traditionally associated with inspiration and creativity in various cultural traditions, though this listing focuses strictly on mineralogical and geological qualities.

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Authenticity & Presentation

This Opal specimen has been carefully chosen to represent the silica alteration processes of the St Austell granite district. 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 granite-related silica mineralisation, this authentic opal specimen combines locality documentation, geological significance and natural matrix presentation in one carefully selected collector’s piece.