minerals in britain

Cut agate from lower Devonianandesite, Ardownie Quarry, Dundee

Minerals inBritain

GemstonesGemstonesBritishGeologicalSurvey

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Although Britain is a major gemstone trader, itcurrently produces little from indigenoussources. There are, however, records of a widevariety of gem material, particularly in

Scotland. In addition to sapphire, ruby and possiblediamond, the country has yielded topaz, beryl, andmany varieties of semi-precious stones includingcairngorm, amethyst, garnet, tourmaline, agate,zircon, ‘Blue John’ flourite and jet. There is also anunconfirmed report of Scottish emerald. Most of thesegemstones were found by amateur geologists and gem-mologists but recent discoveries, particularly ofdiamond indicator minerals in Scandinavia andIreland, have raised commercial interest in thepotential of Britain as a source of precious stones.

Systematic exploration based on historical records, newdatasets and a modern understanding of the geologicalprocesses which control the formation of gemstones canlead to the identification of new prospects. Explorationcan be assisted by computer-based decision-supportsystems, using integrated multidataset analysis ofregional digital parameters, identified and weighted bythe application of the appropriate geological model.Follow-up field-survey verification of prospective areascan be assisted by rapid in-field qualitative analysis ofheavy-mineral concentrates, prior to detailed laboratoryanalysis of mineral suites.

The success of the modern approach to gemstone explo-ration is illustrated by the results from recent commer-cial surveys in Ireland. Little attention was given toprospecting for gemstones in Ireland until a few yearsago, but recent surveys have established the presence ofhitherto unrecorded alluvial sapphires in CountyDonegal and kimberlite indicator minerals in CountyFermanagh, close to where an alluvial diamond was dis-covered in the nineteenth century. Except for a DTI-sponsored Mineral Reconnaissance Programme reporton the diamond potential of Britain, little has beenpublished on gemstone exploration in Britain, butbaseline digital geological, geochemical and geophysicaldatasets are available for study.

DiamondThe eminent Scottish mineralogist Professor M FHeddle, on returning from a visit to the Kimberleydiamond fields of South Africa in the 1870s, undertooka search for diamonds in Scotland, and it is recordedthat he found a small diamond in northern Scotland,4.5 km north-east of Ben Hope. Three supposeddiamond crystals are reported to have been discoveredat the same locality more recently (1958) although, likeHeddle’s claim, this has not been substantiated. In the1890s, crystalline material, thought to be diamond

because it was hard enough to scratch quartz, wasreported from near New Cumnock in southernScotland. The mineral occurred as minute crystals in aseam of graphite, formed by thermal metamorphism ofcoal.

Evidence from the major diamond-producing areas of theworld indicates that most diamonds formed in sub-cratonic roots during the early history of the earth. Inaddition, it is now recognised that diamond could theo-retically form in regions where cold crust is beingsubducted, particularly where the down-going slabcontains carbonaceous sediments. The main require-ments for a primary diamond deposit are therefore:

• A suitable upper-mantle root within the stabilityfield of diamond.

• A host magma formed deep enough to be able topick up fragments of the mantle source region onits way to the surface.

• A structural and tectonic regime for the develop-ment of pipes.

• A rapid transit to the surface in a suitable (kimberlite, lamproite) medium.

Integrated multidataset analysis of regional geological,geochemical and geophysical data.

0 3 km


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Shetland Islands

Cardiff

London

Edinburgh

NewCumnock

Ben HopeLe

wis

ian

Terr

ane

Mull

Arran

Ardnamurchan

Aberdeenshire

Cairngorm

St Just St AustellMoor

Bodmin

Elie Ness

Castleton

Stonehaven

FifePerth

Cheviot Hills

Isle of

Man

Tiree

Loch Glass

Struy Bridge

Strontian

Ayrshire

Whitby

SuffolkCoast

Isle ofWight

Dundonnell

Lincolnshire Coast

BraemarInvercauld

Knoydart

Harris

Southern

Uplands

Glen Buchat

Loch Roag

MamRatagain

GlenUrquhart

DartmoorGranite

N

0 100 200 kilometres

Norfolk Coast

NorthumberlandCoast

YorkshireCoast

Diamond

Sapphire

Ruby

Beryl

Topaz

Cairngorm

Aquamarine

Amethyst

Tourmaline

Agate

Garnet

Zircon

Jet

Blue John

Amber

Torridonian

Land’s End

Dundee

The 1995 Mineral Reconnaissance Programme evaluationof the diamond potential of Britain identified the LewisianTerrane of north-west Scotland as the most prospectivearea, principally because it is a fragment of the ancientLaurentian craton with a lithospheric mantle root and iscut by major shear zones, which seismic data suggestreach the upper mantle. Gem diamond is only known tooccur in kimberlites and lamproites, neither of which hasbeen recorded in this part of Scotland, but exposure ispoor over large areas in north-west Scotland, so it ispossible that pipes have escaped detection.

Fresh kimberlite can contain up to 10% iron oxides, sointrusions may generate distinctive magnetic anomalies,frequently of the order of 200 nT. Most of the existingaeromagnetic cover of north-west Scotland is too widelyspaced to give an adequate indication of whether anydiatremes are present, but a notable and, as yet, unex-plained magnetic anomaly with magnitude c. 600 nTabove background occurs less than 8 km north-north-west of Heddle’s diamond site and, coincidentally, in anarea where alkaline lamprophyre dykes are known tooutcrop.

It has recently been shown by isotopic studies of mineralinclusions that diamonds do not crystallise from kimber-lite or lamproite melts; rather, they occur as mantle-derived xenocrysts and xenoliths, the ultimate sourcebeing garnet lherzolite and/or eclogite forming the wallrocks through which the magma passed. It is thuspossible that other types of intrusions originating in themantle, such as lamprophyres (which have chemical sim-ilarities with kimberlites and lamproites), could alsobring diamonds to the surface. If this is so, the late-Palaeozoic alkaline lamprophyres which occur widelythroughout north-west Scotland offer an additionaltarget for diamond exploration.

Diamonds could also occur in palaeoplacerswithin the undeformed and unmetamor-phosed Proterozoic Torridonian sedimen-tary rocks of north-west Scotland, whichare sourced from an Archaean craton tothe west of the Lewisian terrane.

The hypothesis that diamonds could formin regions of subducted cold crust offersother possible targets for diamond explo-ration, principally in south-west Englandand the Southern Uplands of Scotlandwhere some doubt surrounds the natureof the basement. Rocks which fit into thelamproite class are found in the post-Variscan Exeter volcanic rocks (lateCarboniferous to early Permian) of south-west England and lamprophyres in thesame region approach lamproitechemistry. In addition, aeromagnetic datafor this region, which is more closelyspaced than for the rest of Britain,contains features which may be related tothe presence of unexposed pipe-like intru-sions. In the Southern Uplands of

Scotland some potential exists in lamprophyres, alkalibasalts or nephelinites which locally contain evidenceof mantle origin.

Sapphire and rubyThese gem-quality varieties of corundum occur in moretypes of geological environment than diamond. They arefound in acid pegmatites and basic rocks such as lampro-phyres, and may be associated with included material.They also form by regional and contact metamorphism ofsedimentary rocks, particularly limestone, and arecommonly re-worked into placer deposits.

All of these environments are known in the ScottishHighlands, and the gemstone prospectivity of theregion was confirmed in 1984 when sapphiremegacrysts 10–30 mm in size were discovered in amonchiquite dyke at Loch Roag on the Isle of Lewis. A9.6 carat cut stone from this locality was valued atabout £60 000 in 1995. The dyke is up to 1.5 m wide,cuts Archaean gneisses and contains xenoliths ofmantle-derived spinel-lherzolite. Although a Tertiaryage has been obtained by K/Ar dating, the dyke is nowbelieved to be part of the late-Palaeozoic mantle-xenolith-bearing alkaline lamprophyre suite and haspotential for both diamonds and sapphires.

The existence of sapphires in Scottish rocks has beenknown since the nineteenth century. Small platycrystals have been collected from thermally alteredaluminous sedimentary rocks in Tertiary basaltic andgabbroic rocks on Arran, Mull and Ardnamurchan,and from the Dalradian rocks of Aberdeenshire. Noneof these sapphires are of gem quality, but they provideevidence of potential by indicating the presence ofaluminium-enriched crust favourable for the formationof corundum.


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Left: A 9.6 carat cut sapphire froma lamprophyre (monchiquite) dyke,Loch Roag, Isle of Lewis.

Below: Thin section of the samedyke, showing reaction rimsaround xenocrysts of pyroxene.

Rubies may be present near Dundonnell in theLewisian terrane of north-west Scotland, according toan unconfirmed nineteenth century report and to morerecent claims.

Beryl and topaz Beryl and topaz are typically late-stage products ofgranite magmatism, forming at moderate depths inpegmatites, veins and cavities. They are recorded inseveral Scottish granites, notably Arran andCairngorm where topaz is generally associated withcairngorm. The largest topaz crystals, including someweighing more than 0.5 kg, have come from Beinn a’Bhuird in the Cairngorm mountains.

Beryl crystals up to 7 cm long occur in pegmatites atStruy Bridge, and other occurrences are known in peg-matites from Harris and Knoydart. Alluvial occur-rences are reported from Braemar and Invercauld. Insouth-west England, minor topaz and possibly berylhave been recorded from the Meldon Aplite at themargin of the Dartmoor Granite.

Semi-precious stones Historically, the Cairngorm mountains have beenBritain’s most prolific source of gem material and thesearch for stones formed a locally important industryin the nineteenth century. The stones occur in late-stage pegmatites and drusy cavities in the CairngormGranite and consist principally of yellow to blackcairngorm (a variety of quartz), which may be accom-panied by topaz, aquamarine and beryl. No produc-tion figures survive, but it is recorded that singlecairngorm crystals weighing more that 22 kg werefound. Cairngorm also occurs, with beryl and topaz,

in other granites in north-east Scotland and onArran.

In south-west England, minor occurrences ofcairngorm, amethyst and gem-quality tourmaline areknown in the Dartmoor Granite. Cairngorm andamethyst are also recorded from the southern andeastern margin of the Bodmin Granite but, reputedly,the best of the Cornish amethysts come from St AustellMoor and near St Just and Land’s End.

Gem-quality crystals of elbaite tourmaline up to 40mm long are present in a lithium-enriched granitepegmatite in Glen Buchat, Aberdeenshire. The occur-rence was first documented in 1982 and since then hasproduced many small stones with a variety of huesincluding clear, green, blue green, pink and, rarely,mauve.

Agates are among the most varied and beautiful ofBritish gemstones. They formed in the sites of formergas cavities in andesitic and basaltic lavas or, morerarely, in open cracks and fissures in these rocks. Thebest quality agates are found on the east coast ofScotland between Perth and Stonehaven, in north Fifeand in Ayrshire. They also occur in the Cheviot Hillsand may be found on beaches in several parts ofBritain, including the Isle of Man and theNorthumberland and Yorkshire coasts.

Garnets occur widely in the metamorphic rocks of theScottish Highlands, but most are too fractured, toodark in colour and too included for use as gemstones.However, the pyrope garnets from Elie Ness in Fife,the so called ‘Elie rubies’, have a delicate port-winecolour and are much sought after. These garnets arecontained, along with zircon, in a volcanic vent ofCarboniferous age. Similar vents occur in the westernHighlands.

Zircon is extremely common as an accessorymineral in granitic and metamorphic rocks but

most grains are too small and too impure bevalued as gemstones. However, Heddle

noted several localities, besides Elie, wheregem-quality material might be obtained,including Strontian, Tiree, GlenUrquhart, Struy Bridge, Loch Glass,Mam Ratagain and Ben Hope.

With the exception of south-west England,occurrences of precious and semi-precious

stones are rare in England and Wales. Themost notable worked occurrences are

Whitby Jet and ‘Blue John’ (a variety offluorite). Whitby Jet comprises diagenetically

altered (silicified) araucarian wood occurring asisolated masses in finely laminated dark brown

shales exposed on the Yorkshire coast. Jet was partic-ularly fashionable in Victorian times and its extractiongave rise to a significant local industry. Jet ornamentshave also been produced from several localities inScotland. Derbyshire Blue John comprises purple to


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Colour-banded elbaite tourmaline, Glen Buchat,Aberdeenshire.

Asubstantial amount of minerals-related base-line information for Britain is either publishedor held on open file at the BGS. Increasingly,the data are held in digital form on databases

fronted by a GIS (the BGS MINGOL system) and canbe supplied under licence or as hard-copy products, informats to match the user’s requirements. Some of theprincipal data sets are:

• Mineral Reconnaissance Programme Reports andData Releases.

• Reports, maps and other data provided under theterms of the Mineral Exploration and InvestmentGrants Act 1972 (MEIGA). Some, particularlyairborne geophysical data, have been converted todigital form and can be purchased in user-specifiedformats. Hard-copy data and reports may be pho-tocopied.

• Regional and local scale geochemical surveys.

• Regional and local scale ground and airborne geophysical data coverage.

• Geological mapping at various scales.

• Mineral occurrence and mineral workingsdatabases.

• Drillcore and rock samples, thin sections.

• Planning constraints, licensing and legislativeinformation.

• Minerals trade and production statistics in Britainand worldwide.

• Scientific publications on mineral deposits inBritain.

Staff of the BGS Minerals Programme act as a referencepoint for the supply of advice and information onminerals-related matters in Britain. They can providedetailed information on the above datasets.


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Enquiries should be directed to:The Manager, BGS Minerals Programme, British Geological Survey,

Keyworth, Nottingham NG12 5GG

Tel 0115 936 3494 Fax 0115 936 3520 email [email protected] www.bgs.ac.uk

blue banded colourless to yellow flourite occurring incavities in Lower Carboniferous limestone of theCastleton area. It is said to be unique and, like jet, itgave rise to a local industry which persists today.

Amber has been found in situ at only one locality inBritain, on the Isle of Wight. Itis, however, washed up insmall amounts on the eastcoast of England, mainlyon the Suffolk and Norfolkcoasts but also inLincolnshire. It used to bethought that it was derivedfrom Baltic sources, butpresent evidence suggeststhat it may be derived fromTertiary deposits under theNorth Sea.

The potential to exploit precious and semi-precious stones in Britain has been enhancedrecently by several new gold discoveries, notably insouth-west England and Scotland, as it raises the pos-

sibility of using locally produced gemstones and goldto create jewellery that would command premiumpricing due its rarity and local, historical or royal con-nections.

© NERC 1999. All rights reserved.

Whitby Jet jewellery.

The photographs of sapphire, tourmaline andagate are reproduced with the permission ofthe Trustees of the Royal Museum of Scotland.

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minerals in britain

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