A set of rounded stones shaped like French baguettes have baffled scientists since they were discovered in the high desert near Colorado’s Great Sand Dunes National Park. Researchers assumed they were tools to grind nuts or seeds.

Longmont archaeologist Marilyn Martorano didn’t buy that theory.

The stones were clearly shaped by human hands but didn’t have the right wear marks around the edges to indicate they’d been used for grinding. So she set out to find a better explanation. About a decade later, Martorano believes she’s identified some of the earliest musical instruments ever played in Colorado.

​“You really have to hear them,” said Martorano, who grew up in the San Luis Valley where the dunes sit. “That’s when you believe it.”

Scroll down to for a chance to play the stones for yourself.

Martorano came close to giving up on her investigation. She spent years examining the rocks — one of which has been dated as at least 5,000 years old — without finding a good theory for their use.

A day before she was due to pack up the stones and return them to the Great Sand Dunes National Park & Preserve (the stones are part of their museum collection, but not on display), a friend passed along an online video. It was her light bulb moment.

Brad Turner/CPR News Archaeologist​ Marilyn Martorano shows off a set of lithophones she's studied during a presentation near Longmont.

The video showed a collection of musical stones, or lithophones, from a museum in Paris. Man-made lithophones have been found in Africa, Hawaii, New England and New Mexico. But not around the Great Sand Dunes in Southern Colorado, or the San Luis Valley.

She’d never heard of it before — “Nobody’s ever mentioned it in Colorado,” she said — so she was skeptical.

Once she finished the video, she grabbed some of the Colorado stones she’d spent so much time with and tapped on them.

She heard something resonant. Percussive. Musical.

“And once I tapped them I thought, ‘Wow. Maybe I’m the first person who’s ever heard these in 1,000 years,” she said.

Martorano felt she’d identified a set of ancient musical instruments. She drove to the Great Sand Dunes to share her discovery with a colleague. She asked him to walk to her car, pulled out a few of the stones and asked him to listen. “His mouth just dropped open,” Martorano recalled.