Septarian Nodules: The Dragon Egg Stone Explained

A septarian nodule is a carbonate-rich concretion whose inside is broken into angular blocks by cracks that later filled with minerals. The name comes from the Latin septum, meaning a partition or dividing wall, which is exactly what the mineral-filled cracks look like on a polished face.

The body of the nodule is usually an argillaceous — that is, clay-rich — carbonate, and the cracks are usually filled with calcite, often iron-bearing calcite, together with pyrite and clay minerals. In the Utah material sold in rock shops, a polished slice shows three distinct zones: a grey-brown bentonite and mudstone exterior, a thin band of aragonite lining the cracks, and a bright yellow to honey-coloured calcite fill in the middle.

The nodules themselves started as undersea concretions that grew in marine mud around decaying organic matter. The Utah nodules are Cretaceous in age. When the calcium carbonate from dissolving shells percolated into the cracks, it precipitated as the calcite and aragonite you now see.

There is no settled answer, and a page that gives you one is overstating the science. Four mechanisms are argued in the literature:

• Dehydration shrinkage. The outer shell hardens first while the clay-rich interior dries and shrinks until it pulls apart.
• Syneresis. Colloidal particles pack tighter over time, squeezing out water and contracting the core.
• Pore pressure. Early cementation traps fluid inside, and pressure during burial cracks the body from within.
• Earthquakes. Brittle fracturing from seismic shaking, the most speculative of the four.

What the evidence does suggest is that the cracking happened at shallow burial depth — less than 50 metres, and possibly as little as 12 — not deep in the crust.

Almost every polished septarian egg, sphere or slice on the market is Utah material from the Muddy Creek area of Kane County, near Orderville. A documented specimen in the Fluorescent Mineral Society's database is a polished slice measuring 12 x 1.25 x 14 cm and weighing 346 g — a useful sense of the scale of a typical shop piece.

The words dragon egg and dragon stone are seller names for egg-shaped polished nodules. They are not geological terms and you will not find them in a survey publication; the geological name is septarian concretion.

One genuinely unusual property is worth knowing. The Utah nodules fluoresce, and brightest under longwave ultraviolet. They also phosphoresce — keep glowing after the lamp is switched off — white after shortwave, greenish-white after midwave and yellow after longwave exposure. Both the calcite and the aragonite contribute. If you have a UV lamp, follow its safety instructions and never look into the source or shine it at anyone's eyes.

The three get mixed up constantly, and the difference is easy once you know what to look at.

• A geode is hollow, or was hollow, with crystals that grew inward from a shell towards an open centre. The Kentucky Geological Survey draws exactly this contrast.
• A concretion is solid all the way through and grew outward from a centre, with no crystal-lined cavity.
• A septarian nodule is a concretion that cracked and then had its cracks filled. It is not hollow, so it is not strictly a geode, even though sellers often list it as a septarian geode.

A cut face settles it. Radiating, angular veins that divide the stone into blocks mean septarian. A single open cavity ringed by crystal points means geode.

The yellow fill is calcite, which sits at 3 on the Mohs hardness scale. A copper coin at about 3.5 will scratch it and a steel nail certainly will, so a polished septarian belongs on a shelf, not in a pocket with keys.

Never clean it with vinegar or any acid. Geology labs identify calcite precisely because it fizzes in dilute hydrochloric acid, and household vinegar will etch the polish off the calcite veins while leaving the clay body alone. Warm water, a soft brush and patience is the whole method.

To test a piece you are unsure about, scratch a hidden edge of the yellow vein with a copper coin. If it marks easily, that is consistent with calcite. If nothing marks it, you are not looking at septarian fill.

Septarian concretions are most common in marine shales. Two classic units are the Kimmeridge Clay of England and the Mancos Group of North America. The Moeraki Boulders on the coast of New Zealand are unusually large Paleocene examples of the same kind of concretion, big enough to sit on.

If you are trying to place a specimen, the host rock matters more than the colour. Utah nodules come out of a grey bentonitic mudstone; English and New Zealand material comes out of dark marine shale, and the crack fill tends to be paler and more grey-brown than the strong yellow of the Utah pieces.