Trace fossil, not a mineral

A coprolite is an ichnofossil—evidence of organism behavior, not a body part. It is “what happened,” preserved in stone. Mineral composition depends on burial and groundwater chemistry, so different regions look different.

What is included, what is not

• Coprolite — mineralized feces (fully “cooked” into stone).
• Paleofeces — dried or partially mineralized feces (often archaeological, sometimes still organic).
• Cololite — fossilized intestinal contents inside the body; related but not a discarded “package.”

Phosphatization

Feces are naturally rich in phosphates. In marine and many terrestrial environments, apatite precipitates early, “binding” the mass and preserving fine details (even fragile fish scales or plant tissue fragments).

Carbonate and iron cement

In carbonate-rich waters, the mass can be hardened by calcite (or siderite/iron oxides). Such pieces are usually brown to red and may weakly react with acid.

Silicification

Later fluid flows can silicify the coprolite (chalcedony/jasper), forming polished “agate” interiors with bands and cavities. This is a variety suitable for jewelry, familiar to many.

Palette

• Gray/black — phosphate-rich pieces; sometimes shiny in cross-section.
• Brown/ochre — masses hardened by carbonate or iron materials.
• Reddish red — iron spots; earthy luster.
• Agate orange/peach — chalcedony replacement with bands and cavities.
• Greenish purple — reduced iron or glauconite shades in some outcrops.

Fresh fractures may show embedded particles (bone fragments, shell pieces, plant remains) in contrasting matrix.

Shape glossary

• Spiral — corkscrew/spiral groove shapes, associated with animals having a spiral valve in the intestine (e.g., many sharks, rays, some ancient fish).
• Granules — round or elongated "sausage"-shaped pieces with subtle constriction marks (typical of many vertebrates).
• Segmented — visible "link" banding, sometimes tapering at one end.
• Amorphous — irregular masses, more common in herbivores with many plant remains.

Photo tip: grazing, low-angle (~25–35°) light highlights surface grooves and inclusions. Backlighting silicified pieces makes agate bands sparkle.

Nutritional “confetti”

Look for sharp-edged bone fragments (phosphatic, sometimes porous), shiny fish scale plates (ganoin), small shell shards, seed coats, or plant fibers. They often stand out by color or shine in the matrix.

Matrix hints

Phosphatic matrices appear dense and fine; calcitic ones may show microcrystals in cracks; silicified reveal chalcedony bands and micro-quartz shine.

Surface texture

Sometimes initial grooves, ridges, or spiral furrows remain. Weathering creates a thin crust; a careful fresh break reveals the internal story much clearer.

Concretions (ironstone/carbonate)

May mimic pellets but lack internal nutritional inclusions and show a concentric cement accumulation. Usually homogeneous in cross-section.

Burrow fillings and clay pellets

Fillings of worm/crab burrows or rolled clay lumps resemble feces. They tend to show tubular wall marks or layered sediment, rather than mixed food debris.

Root remains (rhizoliths)

Mineralized roots leave tubular shapes with branching and fibrous texture—not a random mix of food residues.

“Devil’s corkscrew” (Daemonelix)

Often mistaken for a giant spiral coprolite; actually an ancient cave system (fossil rodent/beaver burrows), not feces.

Gastrolith accumulations

Gastroliths are rounded, well-polished pebbles without matrix; coprolites are a bound mass with mixed particles.

Quick checklist

• Are there mixed food inclusions inside? ✔
• Are there consistent shape features (spiral, conical, segments)? ✔
• Does the matrix match local diagenesis (phosphate/calcite/silica)? ✔ → Likely a coprolite.

Where found

Coprolites are known from the Paleozoic to the Cenozoic in marine and terrestrial deposits. They are often found alongside bones and teeth: in coastal phosphate deposits (e.g., river sediments and quarry waste in some southeastern US locations), classic North American and UK dinosaur horizons, and fish-rich formations like the Eocene “Green River” basins. Silicified “agate” coprolites are known from several US Western localities and elsewhere.

Victorian fertilizer boom

Fun fact: In the 19th century, some areas of Cambridgeshire and Suffolk in Britain had “coprolite mining” to extract phosphates for fertilizer. Not all specimens were direct feces—some were phosphatic nodules—but the name stuck, and the fields got fertilized.

Cleaning

• Dry mechanical: wooden picks, bamboo skewers, soft brushes. Compressed air helps blow away dust.
• Avoid acids—they can corrode carbonate cement and sensitive phosphates.
• Briefly soak only sturdy silicified specimens in water with a drop of mild soap; dry thoroughly.

Consolidation

• Strengthen fragile areas with a thin, reversible Paraloid B‑72 (acetone solution), apply sparingly.
• Light polishing is suitable for silicified specimens on display; avoid waxy accumulations that can obscure details.
• Avoid prolonged high humidity and temperature swings—preserves fragile cement.

Exhibition

• Show a cut/polished cross-section alongside the natural specimen—for the interior/exterior story.
• Use neutral backgrounds; raking light for texture, slight backlight—for the agate interior.
• Clear, honest labels turn the "giggle factor" into curiosity and learning.

Backlighting

Hold a thin slice or small polished coprolite against the light. Silicified interiors glow and reveal banding and inclusions like small constellations.

Acid hint (only on chip)

Drop very weak acid on a small chip from a break: fizzing indicates carbonate cement; no reaction—phosphate/silica. Do not test finished surfaces.