The colorful figurine, part of the "Chihuly in the Park" exhibit, was irreversibly damaged Friday night while the conservatory was closed for a private party, Chicago Park District spokeswoman Angelynne Amores said.
"It's like someone plucked the flower," said Amores.
Police said their investigation would include tracking down guests at the party, held to honor a couple on their wedding anniversary. The cost of the damage will be recouped because the party hosts had to carry $2 million worth of insurance.
Police said they didn't know how or why the visitor damaged the sculpture, which looks like a glass vase with a flower and a plant inside.
But police said that given its location in the middle of an ankle-deep pond surrounded by a waterfall whoever broke it had to go to some trouble to get to it.
"You have to crawl through this pond to get to this piece, so obviously it's not totally accidental," said Sgt. John Pallohusky. "But we're just not sure if it was malicious or not."
More than 450,000 people have visited the exhibit since it opened at the conservatory, a large indoor garden house, last November. Because of its popularity, the show was recently extended for a second time until Nov. 4.
Among Chihuly's best-known works are the floral chandelier in the Bellagio hotel-casino in Las Vegas, a 55-foot glass tower at the Oklahoma City Museum of Art, and the newly opened Chihuly Bridge of Glass, a 500-foot pedestrian bridge in Chihuly's native Tacoma, Wash.