Why Chimps Are Captivated by Crystals, and What It Says About Us

Did Nature’s Only Straight Lines Kickstart the Human Mind?

Quartz crystals have been turning up at hominin sites for the better part of a million years. Twenty of them surfaced alongside Homo erectus remains at Zhoukoudian in China, deposited at least 600,000 years ago and possibly earlier. Six nearly complete prisms appeared in an Acheulian layer in India. More showed up at Wonderwerk cave in South Africa, at Gudenushöhle in Austria, in the Kalahari Desert. None of them had been carved, chipped into tools, or strung as ornaments. They were just there, collected and carried home for no reason archaeology has been able to pin down.

So what was the attraction? A team led by Juan Manuel García-Ruiz, a crystallographer at the Donostia International Physics Center in San Sebastián, Spain, decided to put the question to our closest living relatives.

García-Ruiz and colleagues gave two groups of chimpanzees at the Rainfer Foundation in Spain access to crystals of various types, sizes, and optical properties, then watched what happened. The results, published in Frontiers in Psychology, suggest chimps can sort crystals from ordinary pebbles in seconds, fixate on their transparency, and value them enough to hoard them. “We were pleasantly surprised by how strong and seemingly natural the chimpanzees’ attraction to crystals was,” García-Ruiz says. “This suggests that sensitivity to such objects may have deep evolutionary roots.”

The first experiment was blunt. A large transparent quartz crystal weighing 3.3 kilograms, 35 centimetres tall, was placed on a pedestal in the chimps’ outdoor enclosure alongside a brownish sandstone rock of roughly the same size.

Both objects drew the chimps over initially, but the rock lost its charm fast. Yvan, a male in the group, pulled the rock off its base; nobody spent more than two minutes with it. The crystal was a different story. Manuela, the alpha female and strongest member, wrenched it free and carried it to a wooden platform. Within moments Yvan appeared, handled it carefully, and eventually transported it to the dormitories. The group kept it for two days. When caretakers tried to get the crystal back, they had to trade bananas and yogurt for it.

Video footage from the dormitories shows a chimp named Toti sitting on the floor, rotating the crystal in her hands and tilting her head to look through it along its hexagonal axis. The interaction time with the crystal was significantly greater than with the rock (the difference held up statistically, p = 0.0052 on a Kruskal-Wallis test), and all four individuals who could be tracked spent proportionally more time with the crystal than with the rock or the pedestal it sat on.

The second experiment was subtler. Piles of about 20 rounded pebbles were scattered in the outdoor enclosure, each pile seeded with a couple of quartz and calcite crystals. The chimps picked out the crystals within seconds (p < 0.0001). “The chimpanzees began to study the crystals’ transparency with extreme curiosity, holding them up to eye level and looking through them,” García-Ruiz says. Yvan held a 3.5-centimetre quartz crystal close to his eye and examined it repeatedly for more than 15 minutes, with individual bouts of inspection averaging 35 seconds. He isn’t myopic; the team checked.

When pyrite crystals, which are opaque and metallic rather than transparent, were thrown into the mix alongside quartz and calcite, the chimps still picked out crystal-type stones. Sandy, a female in the second group, carried pebbles and crystals in her mouth (chimps don’t typically do this) to a platform where she sorted them into two groups: pebbles on one side, crystals on the other. She separated quartz, pyrite, and calcite from the lot, despite the three differing in transparency, symmetry, and lustre. “This ability to recognize crystals despite their differences amazed us,” says García-Ruiz.

So it seems both transparency and geometric shape pulled the chimps in. And the researchers reckon there’s something deeper going on here than novelty-seeking. Look around a savannah, a forest, pretty much any natural landscape, and what you see is curvature and fractal branching: trees, clouds, rivers, animal bodies. Straight lines and flat planes are vanishingly rare. Crystals are, in fact, the only naturally occurring polyhedral objects. Everything else with Euclidean geometry, every flat wall and sharp corner, was invented by us. When early hominins stumbled across a quartz prism in the dirt, they were holding something unlike anything else in their world.

That strangeness could have consequences well beyond simple curiosity. The collection of crystals by hominins roughly coincides with improving symmetry in stone hand axes around 500,000 years ago. The paper speculates (carefully) that encountering natural polyhedra might have nudged early minds towards recognising geometric patterns and, eventually, towards abstraction itself. Did crystals help catalyse the neuronal processes behind abstract thought? García-Ruiz poses the question without claiming to answer it.

There are caveats, and the team is upfront about them. Nine chimps is a small sample. These are enculturated animals, raised around human objects with flat surfaces and straight edges; we can’t know if wild chimps would respond the same way. Personality matters too. “There are Don Quixotes and Sanchos: idealists and pragmatists,” García-Ruiz says. “Some may find the transparency of crystals fascinating, while others are interested in their smell and whether they’re edible.” Future work needs wild apes, bonobos, gorillas. But the pattern is there, at least provisionally: hand a chimp a crystal and something kicks in, something that perhaps kicked in for Homo erectus staring at a quartz prism in a cave 780,000 years ago. “We now know that we’ve had crystals in our minds for at least six million years,” says García-Ruiz.

Study link: https://doi.org/10.3389/fpsyg.2026.1633599