Sea urchin spines are extraordinary skeletal structures made almost entirely of a single crystal of calcite—a form of calcium carbonate—yet they behave like flexible armor.

While most people imagine spines as needle-sharp, many species have rounded, club-like, or even hair-thin forms. For example, the purple sea urchin (Strongylocentrotus purpuratus) sports slender, pointed spines used for defense and movement, while species like Tripneustes gratilla have smooth, blunt spines adapted for burrowing and gentle handling of seagrass.

These spines grow from the animal’s test (shell) via living tissue that deposits layers of calcite with remarkable precision, creating a material five times stronger than chalk. Each spine tells a story of ocean engineering—microscopic lattices turned into armor through pure crystal growth.

Includes 3 thick sea urchin spines with an info card. These ethically sourced urchin spines were collected from naturally expired specimens, ensuring no live urchins are harmed.