Stromatolite — Layered Life Written in Stone
Stromatolites are Earth’s hand‑written history: thin, repeated layers built by microbial mats—often cyanobacteria—that trap grains and encourage minerals to precipitate. Each lamina is a page; stack thousands and you get domes, columns, and rippling sheets that record ancient shorelines, changing waters, and the long arc of early life. They’re beautiful to look at and even better to learn from.
Identity & Origins 🔎
Microbial architecture
Stromatolites are built by microbial mats (often cyanobacteria with other microbes). The mats produce sticky films that trap sediment and their photosynthesis can raise local pH, causing carbonate to precipitate. The mat then grows upward and repeats, making unmistakable laminations.
Deep time, living present
They’re among the oldest conspicuous 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 grazing—think hypersaline lagoons and alkaline lakes.
How They Grow 🧭
Step 1 — Sticky start
A microbial mat colonizes a surface, traps silt and sand with extracellular films, and smooths the surface.
Step 2 — Mineral paint
Photosynthesis shifts local chemistry. Carbonate precipitates (in marine/lake settings), or silica later replaces the fabric during silicification.
Step 3 — Upward & repeat
New mat growth climbs above the last layer, repeating the cycle. Environmental energy shapes the form: planar in calm waters, domal/columnar where currents and light vary.
Imagine a living paint roller: roll, dust, mineralize—then roll again. The canvas is time.
Palette & Pattern Vocabulary 🎨
Palette
- Cream to bone — carbonate laminae.
- Greys — silicified/cherty replacements.
- Ochre & russet — iron staining.
- Sage & olive — chloritic/clay notes in altered dolostone.
- Charcoal — organic‑rich laminae, iron oxides.
Polish ranges from matte‑satiny (dolostone) to vitreous (silicified). Thin slices glow with banded translucency.
Pattern words
- Lamination — microlayers like tree‑ring lines.
- Domal/columnar — hemispherical stacks and vertical pillars.
- Conical (conophyton) — steep, nested cones.
- Oncoid — marble‑to‑golf‑ball sized spherical coated grains with concentric rings.
- Fenestrae — tiny voids/patches between laminae.
Photo tip: Raking light at ~25–35° makes microlaminae pop; a small backlight behind a thin slice turns it into striped stained glass.
Physical Details 🧪
| Property | Typical Range / Note |
|---|---|
| Nature | Biosedimentary structure (not a single mineral); composition inherits the host rock |
| 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 (carbonate/dolostone) • ~2.6 (silica) |
| Luster / Transparency | Matte to vitreous; opaque to translucent on thin edges if silicified |
| Fracture / Cleavage | Carbonates show perfect cleavage in rhombs (visible in calcite veins); silicified material breaks conchoidally |
| Reaction to acid | Calcite laminae effervesce in dilute acid; silicified layers do not |
| Treatments | Occasional stabilization of porous slabs; sometimes color‑enhanced low‑grade material—request disclosure |
Under the Loupe 🔬
Microlaminae
Tightly spaced lines with slightly different grain sizes or mineralogy. In carbonate hosts, laminae may alternate between micrite (fine mud) and spar (clear calcite).
Trapped grains
Quartz silt and tiny peloids “caught” in sticky mat layers—evidence of trapping/binding. Silicified pieces show a micro‑mosaic of quartz.
Fenestrae & seams
Small voids, later filled by calcite or silica, and wispy seams that mark pauses in growth—excellent clues to environment.
Look‑Alikes & Mix‑ups 🕵️
Travertine / onyx marble
Also banded carbonates but formed by inorganic spring deposition; bands are thicker, often with cavernous voids and botryoidal crusts rather than microlaminae.
Banded jasper/chert
May show rhythmic stripes, but lack clear domal/columnar architecture and the fine, uniform microbial laminations.
Fossil coral & algae
Corals show honeycomb or radiating septa; calcified algae can be concentric but more cellular. Stromatolites read as continuous layers.
Turritella “agate”
Packed spiral shells (really Elimia snails) in chalcedony—very different from layered mats, but often compared in the shop.
Quick checklist
- Regular, fine laminations?
- Domes/columns or concentric oncoids?
- Environmental story visible (trapped grains, filled voids)? → Stromatolite.
Localities & Lapidary 📍
Where it shines
Ancient stromatolitic rocks are widespread. Collector favorites include silicified stromatolite 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 certain Bahamian tidal flats.
What people make
Cabochons that reveal rolling laminae, spheres & bookends from massive blocks, tabletops that look like topographic maps, and thin backlit slices that turn galleries into mini‑museums.
Care & Display Notes 🧼🪨
Everyday care
- Silicified pieces: sturdy—clean with mild soap + water; avoid hard knocks at edges.
- Carbonate hosts: keep away from acids/bleach; use only mild soap + water and a soft cloth.
- Porous zones benefit from gentle handling; avoid prolonged soaking.
Lapidary tips
- Map laminae before cutting; orient to maximize rolling band contrast.
- Stabilize porous seams if needed and disclose.
- Finish: diamond pre‑polish; cerium/tin oxide on leather/felt for glassy silicified pieces; lighter touch on carbonates.
Display & photography
- Raking light reveals micro‑relief of laminae.
- Backlight thin slices for a dramatic, map‑like effect.
- Pair a polished face with a raw fragment to show “from mat to monument.”
Hands‑On Demos 🔍
Lamination spotlight
Shine a narrow beam across the specimen at a low angle. Count how many laminae per millimetre you can see—then imagine that repeated for millennia.
Host test (for a spare chip)
Place a single drop of dilute acid on a spare fragment: fizz = carbonate host; no fizz & glassy fracture = silicified. Handy for care guidance.
It’s a time‑lapse of life and sediment, pressed into a palm‑sized sculpture.
Questions ❓
Is stromatolite a fossil or a rock?
It’s a biosedimentary structure—a rock fabric built by living mats. Many consider it a type of trace fossil of microbial activity.
Why do some pieces look ringed like tree slices?
Those are oncoids—stromatolite cousins that grew as spherical coated grains rolling on the seafloor.
Does color equal age?
No. Color reflects minerals (carbonates, silica, iron oxides) and subsequent alteration, not the absolute age of formation.
Good for daily‑wear jewelry?
Silicified stromatolite fares best; carbonate hosts are softer and prefer gentle, occasional wear.