"Rock of the Month # 269, posted for November 2023" ---

THE NATURE OF AGATE

Agate, with its attractive, variable appearance, superior polishing qualities and durability, is a favourite with the lapidary and jewellery crafts, and is featured in many a handbook of minerals and gems (e.g., Metz, 1965; Desautels, 1968; Kirkaldy, 1968; Sinkankas, 1976; Bonewitz, 2005). Fine-grained, gemmy silica is often referred to as chalcedony, agate being a banded form. Under the microscope chalcedonic silica often shows a fibrous structure.

Because of its toughness and ability to be worked into a cutting edge, silica minerals (like the more-abundant cryptocrystalline forms known as chert or flint) have been used for millennia in the production of tools and weapons. Thus chert, agate and jasper are amongst the constituents of stone tools described by generations of archaeologists in India (Foote, 1916) and elsewhere. Barbara Luedtke (1992) reviewed the importance of chert in archaeology, where the provenance of lithic artefacts is of great importance for the identification of ancient trade routes. While chert is a specific form of fine-grained silica found largely in carbonate host rocks (described in several previous Rocks of the Month), for students of stone tools a broad definition of chert might include all sorts of varieties of sedimentary rocks dominated by microcrystalline quartz, including flint, chalcedony, agate, jasper, hornstone and novaculite, and these she describes in great detail.

OCCURRENCE

Agate is generally precipitated in voids and fissures. The semi-massive silica is much more chemically and mechanically resistant than the host rocks, typically lava flows, and so the infilled voids weather out to become loose geodes on the surface. The stones can roll about for relatively long periods, to be found by the observant along rivers and beaches, as described in England by Ellis (1957), a work that influenced me greatly, when presented by said uncle John on my 9th birthday!

In some regions, such as southern Brazil and neighbouring parts of Paraguay, Uruguay and Argentina, where Jurassic-Cretaceous basalt flows yield much agate, the material can be collected in abundance (Balcom, 1974). The association of lava flows with void-filling material such as agate and zeolites was noted long ago in basalt provinces, such as the Deccan of India (Blanford, 1867) and the lower Ashangi Group in Ethiopia, which includes amygdaloidal basalt with agate and zeolite, often coated with green earth (the zeolite is usually white or orange stilbite). In geologically-young Iceland, too, amygdales (amygdules) in lavas are a source of many minerals, such as zeolites and agate (Saemundsson and Gunnlaugsson, 2014) and that country yields many forms of silica (ibid., pp.134-153).

Agate is very widespread, as noted by Frederick Cornell in his epic quest for diamonds and other minerals across South Africa and Namibia (Cornell, 1920). Forms of quartz were amongst many mineral varieties known in Antiquity, including forms such as iaspis (in part green quartz), rock crystal, amethyst and chalcedony, agate, xanthe (yellow jasper) and chert (Caley and Richards, 1956).

ORIGINS

The many kinds of silica are described by Frondel (1962). Agate is crystalline (or technically maybe cryptocrystalline) silica, containing fine silica fibres, as noted in a study of banded agate from Rio Grande do Sul, Brazil (Wang and Merino, 1990). The silica is thought by many to precipitate from a gel (Frondel, 1962, pp.150-158). However, disparate crystal habits suggest different crystallization mechanisms for quartz and chalcedony, and it is thought that the latter can form at low temperatures, <100°C. Heaney (1993) proposed a model in which the precipitation of silica may occur direct from solution, and not via a gel stage. Analyses of oxygen isotopes in silica and hydrogen isotopes in bound water in Scottish agates of two ages (Fallick et al., 1985) indicated that the water is genetically significant in agate formation. The agates (from the lower Devonian, and from the Tertiary lavas of Mull), formed at about 50°C from fluids having at least a component of meteoric origin. Lastly, while the great majority of material termed agate seems to be formed via precipitation from fluids in voids, and especially in cooling but recently-plastic lava flows (hence the flattened amygdales), some is clearly generated by replacement in the solid state, with silica supplanting pre-existing minerals. Replacement of fossils is a clear example of this.

VARIETIES

I am not going to compile a detailed list - the lapidary and gemstone literature is rife with agate references - but the following are a selection of varietal names of agate finds and sales since the year 2000. Whereas there are some old names that have stuck, such as moss agate and blue lace agate, many others are surely destined to be short-lived, of definite but transient commercial value. An exception may be the purple, botryoidal grape agate from Sulawesi, Indonesia. Many varieties have been recovered across Mexico (Cross, 2008, mentions at least 20 of them).

• Anatolian blue agate (Turkey: Landon, 2020a)
• Brachiopod agate (Jemez Mountains, New Mexico: with silicified brachiopods such as Composita subtilita; Burger, 2011).
• Chinese rain flower agate (Perkins, 2012)
• Crazy lace agate (Thompson, 2018).
• Disdero agate (east of Kamloops, British Columbia: Landon, 2013).
• Ellensburg blue agate (Oregon, formed in fractures in the Teenaway basalt formation; Landon, 2020b)
• Grape agate (Sulawesi; Ivey, 2018).
• Mexican Laguna lace agate (Landon, 2020c)
• Plume agate (eastern Oregon, Thompson, 2012).
• Purple sagenitic agate (western USA: see Halas, 2011).

REFERENCES

Balcom,NW (1974) A rock trip through the agate country of Brazil and Argentina. Lapidary J. 28 no.7, 1146-1150, October.

Blanford,WT (1867) On the traps and and intertrappean beds of western and central India. Geol.Surv.India Vol.6, 137-162, reprinted in Deccan Volcanic Province (Subbarao,KV editor), Geol.Soc.India Memoir 43(1), 2538, 1999.

Blanford,WT (1870) Observations on the Geology and Zoology of Abyssinia. Made during the Progress of the British Expedition to that Country in 1867-68. Macmillan and Co., London, 487pp., reprinted in 2005 by Adamant Media Corporation in the Elibron Classics Series.

Bonewitz,RL (2005) Smithsonian Rock and Gem. DK Publishing Inc. / Smithsonian Institution and Dorling Kindersley Limited, 360pp.

Burger,F (2011) Brachiopod agate. Lapidary Journal Jewelry Artist 65 no.1, 26-27.

Caley,ER and Richards,JFC (1956) Theophrastus on Stones. Ohio State University, Columbus, OH, 238pp.

Cornell,FC (1920) The Glamour of Prospecting. Wanderings of a South African Prospector in Search of Copper, Gold, Emeralds, and Diamonds. David Philip, Publisher (Pty) Ltd, Cape Town, 1986 edition, 336pp.

Cross,BL (2008) Classic agate deposits of northern Mexico. Mineral.Record 39 no.6, 69-88.

Desautels,PE (1968) The Mineral Kingdom. Ridge Press, Inc. / Madison Square Press / Grosset & Dunlap, New York, 252pp.

Ellis,C (1957) The Pebbles on the Beach. Faber and Faber Limited, London, 2nd edition, 163pp.

Fallick,AE, Jocelyn,J, Donnelly,T, Guy,M and Behan,C (1985) Origin of agates in volcanic rocks from Scotland. Nature 313, 672-674.

Foote,RB (1916) Prehistoric and Protohistoric Antiquities of India. Leeladevi Publications, Delhi, 246pp. plus 64 plates, 1979 reprint.

Frondel,C (1962) The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana. Volume 3, Silica Minerals. John Wiley and Sons, Inc., New York, 7th edition, revised and enlarged, 334pp.

Halas,R (2011) Purple sagenitic agate. Lapidary Journal Jewelry Artist 65 no.1, 48-49.

Heaney,PJ (1993) A proposed mechanism for the growth of chalcedony. Contrib. Mineral. Petrol. 115, 66-74.

Ivey,J (2018) "Grape agate" from West Sulawesi, Indonesia. Mineralogical Record 49 no.6, 827-836.

Kirkaldy,JF (1968) Minerals and Rocks in Colour. Blandford Press, London, 2nd edition, 184pp.

Landon,J (2013) BC's new agate. Lapidary Journal Jewelry Artist 67 no.1, 56-58.

Landon,J (2020a) Something old, new, gemmy & blue. Lapidary Journal Jewelry Artist 74 no.1, 52-54.

Landon,J (2020b) Ellensburg blue. Lapidary Journal Jewelry Artist 74 no.3, 38-41.

Landon,J (2020c) The right rough. Lapidary Journal Jewelry Artist 74 no.4, 76-79.

Luedtke,BE (1992) An Archaeologist's Guide to Chert and Flint. Archaeological Research Tools 7, Institute of Archaeology, University of California, Los Angeles, 172pp.

Metz,R (1965) Gems and Minerals in Color. Hippocrene Books, New York, Eng.Trans. 1974, 255pp.

Perkins,J (2012) Chinese rain flower agate. Lapidary Journal Jewelry Artist 66 no.7, 62-63.

Saemundsson,K and Gunnlaugsson,E (2014) Icelandic Rocks & Minerals. Mal og Menning, Reykjavik, 2nd edition, 232pp., translated from the Icelandic by Anna Yates, with photography by G. Eiriksson

Sinkankas,J (1976) Gemstones of North America, Vol. II. Van Nostrand Reinhold, 494pp.

Thompson,SE (2012) Plume agate. Lapidary Journal Jewelry Artist 65 no.9, 20-23.

Thompson,SE (2018) Crazy! Lapidary Journal Jewelry Artist 72 no.1, 20-21.

Wang,Y and Merino,E (1990) Self-organizational origin of agates: banding, fiber twisting, composition, and dynamic crystallization model. Geochemica et Cosmochimica Acta 54, 1627-1638.