Stromatolite — layered life recorded in stone
Stromatolites are Earth's handwritten history: thin, repeating layers grown by microbial mats—often cyanobacteria—that trap grains and promote mineral deposition. Each lamina is like a page; stack thousands and you get domes, columns, and undulating slabs recording ancient shores, changing waters, and a long trajectory of early life. They are beautiful to look at and even better to learn from.
Identity and origins 🔎
Microbial architecture
Stromatolites are formed by microbial mats (often cyanobacteria with other microbes). The mats secrete sticky films that trap sediments, and photosynthesis can raise the local pH, causing carbonates to precipitate. Then the mat grows upward and repeats the process, leaving unmistakable laminations.
Ancient past, living present
These are among the oldest clear biosedimentary structures in the rock record (well-studied examples older than 3.45 billion years). Yet they still grow today in a few places where conditions limit erosion — e.g., hypersaline lagoons and alkaline lakes.
How it grows 🧭
Step 1 — sticky start
A microbial mat colonizes the surface, extracellular films trap algae and sand and smooth the surface.
Step 2 — mineral “paints”
Photosynthesis changes local chemistry. Carbonates precipitate (in marine / lake settings), or later the fabric is replaced by silicification.
Step 3 — up and repeat
A new mat layer grows over the previous one and the cycle repeats. Environmental energy shapes the form: flat in calm waters, domal / columnar where currents and light vary.
Imagine a living paint roller: rolling, sprinkling, mineralizing — and rolling again. The canvas is time.
Palette and pattern dictionary 🎨
Palette
- Cream to bone color — carbonate laminae.
- Gray — silicified / chert replacements.
- Ochre and rust — iron staining.
- Sage and olive — chloritic / clay impurities in altered dolostone.
- Carbonaceous — organic-rich laminae, iron oxides.
Polish ranges from matte–silky (dolostone) to glassy (silicified). Thin sections shine with banded translucency.
Pattern words
- Lamination — microscale layers like tree ring lines.
- Domal / colonial — hemispherical mounds and vertical “columns.”
- Conical (conophyton) — steep, embedded cones.
- Onkoids — spherical coated grains ranging from marble-sized to golf ball-sized with concentric rings.
- Fenestrations — small cavities/spots between laminae.
Photo tip: Side, "grazing" light at ~25–35° angle highlights microlaminae; a thin slice illuminated from behind turns into a striped stained glass.
Physical details 🧪
| Property | Typical range / note |
|---|---|
| Nature | Biosedimentary structure (not a single mineral); composition inherits main rock properties |
| Common compositions | Carbonate (calcite/aragonite/dolomite), silicified (chert/jasper), locally iron-rich (banded iron formations) |
| Hardness (Mohs) | ~3 calcite • ~3.5–4 dolomite • ~6.5–7 silicified |
| Specific gravity | ~2.6–2.9 (carbonates / dolostone) • ~2.6 (silica) |
| Luster / transparency | From dull to glassy; opaque, but silicified material is translucent at thin edges |
| Fracture / cleavage | Carbonates show perfect rhombohedral cleavage (visible in calcite veins); silicified material breaks conchoidally |
| Reaction with acid | Calcite laminae fizz with diluted acid; silicified ones do not |
| Treatments | Sometimes stabilized porous plates; color enhancement occurs in lower quality material — request exposure |
Under the magnifier 🔬
Microlaminae
Densely arranged lines with slightly varying grain size or mineralogy. In carbonate rocks, laminae may alternate between micrite (fine mud) and spar (transparent calcite).
Trapped grains
Quartz dust and tiny peliods "caught" in sticky layers — evidence of trapping and binding. Quartz micromosaic is visible in silicified pieces.
Fenestrations and seams
Small cavities later filled with calcite or silica, and very thin seams marking growth pauses — excellent environmental traces.
Similar and confusing 🕵️
Travertine / onyx marble
Also banded carbonates, but formed by inorganic source sedimentation; bands are thicker, frequent cavity voids and botryoidal crusts, not microlaminae.
Banded jasper / chert
May have rhythmic bands but lack clear dome / columnar architecture and uniform microbial-origin laminae.
Fossilized corals and algae
Corals show honeycomb or radial septa; calcareous algae can be concentric but are more cellular. Stromatolites count as continuous, uninterrupted laminae.
"Turritella" "agate"
Densely packed spiral snail (actually Elimia) shells in chalcedony — quite different from layered mats, but often compared in stores.
Quick checklist
- Regular, fine laminations?
- Domes / columns or concentric oncoids?
- Visible environmental history (trapped grains, filled cavities)? → Stromatolite.
Localities and processing 📍
Where it shines
Ancient stromatolitic rocks are widespread. Collectors' favorites: silicified stromatolites from Australia and North America, Kona dolomite (stromatolitic dolostone) from Michigan, and iron-rich stromatolitic layers in some banded iron formations ("Mary Ellen" type jaspers). Modern analogs grow in places like Shark Bay (Western Australia) and some Bahamian tidal flats.
What they create from them
Cabochons revealing rolling laminae, spheres and bookends from massive blocks, countertops resembling topographic maps, and thin backlit slices that turn galleries into mini museums.
Care and display notes 🧼🪨
Daily care
- Silicified pieces: sturdy — clean with mild soap and water; protect edges from impacts.
- Carbonate substrates: keep away from acids/bleach; only mild soap + water and a soft cloth.
- Porous zones prefer gentle handling; avoid prolonged soaking.
Processing tips
- Mark the laminae before cutting; orient so that the contrast of the "wavy" bands is maximal.
- If needed, stabilize porous seams and reveal them.
- Finish: diamond pre-polish; cerium / tin oxide on leather/felt for glassy silicified pieces; gentler touch for carbonates.
Exposure and photography
- “Grazing” light reveals laminae microrelief.
- Backlight a thin slice — dramatic, map-like effect.
- Pair the polished face with the green fragment — “from mat to monument” in one composition.
Practical demonstrations 🔍
Lamination highlight
Shine a narrow beam of light at a low angle through the specimen. Count how many laminae per millimeter you see — then imagine it lasted millennia.
Base test (for spare chips)
Drop one drop of diluted acid on a separate fragment: fizzing = carbonate base; no fizz and glassy fracture = silicified. Great for care recommendations.
It is a slowed film of life and sediment, compressed into a palm-sized sculpture.
Questions ❓
Is a stromatolite a fossil or a rock?
This is a biosedimentary structure — rock fabric created by living mats. Many consider it a trace fossil of microbial activity.
Why do some pieces look ringed like a tree cross-section?
These are oncoids — stromatolite cousins, grown as spherical coated grains rolling on the sea floor.
Does color indicate age?
No. Color is determined by minerals (carbonates, silica, iron oxides) and later alterations, not absolute formation age.
Is it suitable for everyday jewelry?
Silicified stromatolite holds up best; carbonate bases are softer and occasionally wear gently.