A new study published in Frontiers in Psychology offers a possible answer. Researchers in Spain explored what makes crystals so appealing by studying how chimpanzees, one of our closest living relatives, respond to them. Their experiments suggest that the attraction to crystals may have deep evolutionary origins that predate modern humans.

"We show that enculturated chimpanzees can distinguish crystals from other stones," said lead author Prof Juan Manuel García-Ruiz, an Ikerbasque Research Professor on crystallography at the Donostia International Physics Center in San Sebastián. "We were pleasantly surprised by how strong and seemingly natural the chimpanzees' attraction to crystals was. This suggests that sensitivity to such objects may have deep evolutionary roots."

Chimpanzees Show an Unexpected Attraction to Crystals

Humans and chimpanzees split from a common ancestor between six and seven million years ago, and the two species still share many behavioral and genetic traits. To investigate whether an attraction to crystals might be one of them, researchers gave two groups of enculturated chimpanzees at the Rainfer Foundation access to crystals. The first group included Manuela, Guillermo, Yvan, Yaki, and Toti. The second group included Gombe, Lulú, Pascual, and Sandy.

In the first experiment, the researchers placed a large crystal, known as the monolith, beside a regular rock of similar size. Both objects initially caught the chimpanzees' attention, but the crystal quickly became the clear favorite while the ordinary rock was largely ignored.

After taking the crystal from the platform, the chimpanzees carefully examined it, rotating and tilting it to view it from different angles. One chimpanzee, Yvan, eventually picked up the crystal and deliberately carried it back to the dormitories.

The researchers found that the animals' curiosity was strongest immediately after they encountered the crystal and slowly faded over time, a pattern similar to the way novelty wears off in humans. Caretakers later had to trade bananas and yogurt to persuade the chimpanzees to return the crystal.

Chimps Quickly Picked Out Crystals From Ordinary Stones

A second experiment tested whether the chimpanzees could recognize smaller quartz crystals similar in size to those collected by ancient hominins. Mixed into a pile of 20 rounded pebbles, the crystals were selected within seconds.

Even after the researchers added pyrite and calcite crystals, each with different shapes from quartz, the chimpanzees still recognized and selected the crystal type stones.

"The chimpanzees began to study the crystals' transparency with extreme curiosity, holding them up to eye level and looking through them," García-Ruiz said.

The animals continued examining the crystals for hours.

One chimpanzee named Sandy displayed particularly interesting behavior. She carried both pebbles and crystals in her mouth to a wooden platform, where she sorted them into separate groups.

"She separated the three crystal types, which themselves differed in transparency, symmetry, and luster, from all the pebbles. This ability to recognize crystals despite their differences amazed us," García-Ruiz said.

Chimpanzees do not normally transport objects in their mouths, so the researchers suggest Sandy may have been concealing the crystals, behavior that could indicate she regarded them as valuable.

What Crystals May Have Meant to Early Humans

The study did not investigate whether certain chimpanzees were naturally more interested in crystals than others, although the researchers say future work should consider differences in personality.

"There are Don Quixotes and Sanchos: idealists and pragmatists. Some may find the transparency of crystals fascinating, while others are interested in their smell and whether they're edible," García-Ruiz pointed out.

The researchers also noted that the chimpanzees in this study are accustomed to living around humans and regularly encounter objects that do not exist in the wild. They say similar experiments should eventually be carried out with less enculturated apes, ideally wild populations.

By combining the observations from both experiments, the team concluded that transparency and geometric shape appear to be the features that make crystals especially attractive. Those same qualities may also explain why early humans chose to collect them.

Unlike trees, mountains, rivers, clouds, and animals, which are dominated by curved and branching forms, crystals stand out because of their flat surfaces and straight edges. They are the only naturally occurring polyhedral solids, making them unlike almost anything else in the natural landscape. The researchers suggest that these unusual patterns may have captured the attention of early humans as they tried to understand the world around them.

"Our work helps explain our fascination with crystals and contributes to the understanding of the evolutionary roots of aesthetics and worldview," concluded García-Ruiz. "We now know that we've had crystals in our minds for at least six million years."