The unique pattern within this black mineral concretion is a one-of-a-kind geologic formation. These stones, known as septarian nodules, have been internally cracked and filled with white calcite crystal over millions of years. Also nicknamed the “dragon’s egg geode," the concretion is a great example of the beauty that can be found in the natural world.

Geologists are unsure of the exact formation process of the cracked lightning pattern, but it is theorized that dehydration of the stone may cause it to shrink and crack.

This specimen is a septarian nodule which has been split to reveal the cracked pattern within.

Both Small and Large-sized septarian nodules are available. The small specimens measure about one inch in diameter, while the larges are around 2 to 3 inches long. Both specimen sizes ship in a sturdy shipping carton as a split pair, allowing you to open and share this natural curiosity with any visitor. These pairs are matched to one another, so you can see the pattern reflected when the stone is opened.

Each septarian nodule comes with an informational photo card that also serves as certificate of authenticity.

Septarian nodules are an intriguing geologic phenomenon with an instantly recognizable look. They are something of a misnomer as they are actually concretions, material built up around a central core, rather than nodules, a replacement body around a contrasting mineral.

These stones are identified by two fractures, an internal radial breakage and a concentric one appearing on the surface. The internal cracks are filled in with an outside material, oftentimes white calcite crystals. Because of these striking formations, septarian nodules are often known by more fanciful nicknames, such as the dragon’s egg geode, beetle stone, and turtle stone.

Like all concretions, septarian nodules form from sediment accumulating and cementing around a nucleus (such as a fossil), forming a rough sphere shape. Septarian nodules usually form alongside coastlines and riverbanks—it is theorized that the movement of water may aid in their formation. In terms of size, septarian nodules have an average diameter between one and 15 centimeters, though some have been found at three meters long.

How the cracks form is still debated. One theory is that dehydration of the stone may cause it to shrink and crack, but it is also thought that the breakdown of the organic material within the concretion can explain the cracks. Besides calcite, septarian nodules cracks have also been found filled with gypsum, aragonite, and barite.

Septarian nodules get their name from their distinctive cracked patterns. These cracks are also called septarium, whose etymology comes from “septum” the Latin word meaning enclosed space, or dividing wall.

The cracks form unique patterns across each septarian nodule, molded by different minerals to create striking patterns that are never repeated again.