Ornamental "fortification agate"

a lapidary favourite

geode exteriors [433 kb] geode interiors [289 kb]

Fig. 1: Two modest yet attractive geodes (nodules) of banded agate, shown in exterior view and (right) sawn and polished interiors. Locality unknown, but suspected to be from the American Southwest (I have no firm evidence for this!). These two little agate geodes were bought, for the princely sum of US$2 apiece, from the American Museum of Natural History in New York, on 05 March 1983 (GCW numbers 114 and - more elongate piece - 115).

"Rock of the Month # 277, posted for July 2024" ---

Agate, and the many other related forms of silica (quartz: silicon dioxide plus colouring agents such as iron oxides) is one of many minerals known since ancient times (Boyle, 2024, pp.309, 431). The varieties with concentric bands, reminiscent of the walls and keep of an ancient castle, are sometimes termed "fortification agate". This form occurs in, e.g., the Lake Superior region of the U.S.A. (Lynch and Lynch, 2008) and in Iowa (Eckert, 1987). Visual inspection suggests that the geodes are basically all silica, from outermost "rind" to the core, though the colour banding speaks to likely variation in levels of impurities such as iron.

Agate geodes

Each geode shows evidence of a two-stage history. Here is one possible mode of formation. Each most likely began life as a void ("gas bubble") in a cooling lava flow, a "negative feature" if you will. The first infill may have occurred while the parent voids were on permeable fractures, such that siliceous fluids could pass through the flow and precipitate out, the fluid composition evolving over time. Such filling accounts for some 15% of elongate geode 115, and about 70% of 114. At this point in the development of each geode, perhaps the feeder fractures became sealed, and the fluid in the diminished voids crystallized from the walls inward, forming a concentric pattern. The centre of each filling appears to be more coarsely crystalline, the final stage of geode development.

Silica-rich nodules, some superficially resembling geodes, can also form in sedimentary settings, either as near-pure silica or in combination with other minerals, in sedimentary settings (e.g., chert, as in the famous flint nodules in the Chalk of northwest Europe). Dissolution of the carbonate host may be required prior to precipitation of silica, or void space may already be available (animal burrows, bedding planes, fault planes, collapse zones, etc).

For a more detailed account of some agate varieties, see the entry from last November on "Agate, colourful silica".


Boyle,RW (2024) A History of Geochemistry and Cosmochemistry, from Prehistory to the end of the Classical Period. Cambridge Scholars Press, Newcastle upon Tyne, England (Wilson,GC, Butt,CRM, Garrett,RG and Robinson,H, editors), 50+580pp.

Eckert,AW (1987) Earth Treasures Volume 3, the Northwestern Quadrant. Harper and Row Perennial Library, 632pp.

Lynch,B and Lynch,D (2008) Rocks & Minerals: A Field Guide to the Lake Superior Area. Adventure Publications Inc., Cambridge, MN, 208pp.

Graham Wilson, 01-03 July 2024 (with minor edit from T.J. Barrett!)

For further information, see:

Rock of the Month Thematic Index

or, visit the Turnstone "Rock of the Month" Chronological Archives!