A Colorado woman visiting Arkansas' Crater of Diamonds State Park this week discovered an 8.66-carat white diamond, the third-largest stone unearthed by visitors since the park – the only publicly-owned diamond prospecting site in the world – opened its doors in 1972.

Park Superintendent Justin Dorsey said that nearly 30 years had passed since the park had yielded a diamond of comparable size – the 8.82-carat Star of Shreveport, which was discovered in 1981. The largest diamond found to date at the park is the 16.37-carat Amarillo Starlight, which was unearthed by a visitor from Texas in 1975.

The lucky prospector, Beth Gilbertson, said that when she first noticed the stone in her bucket, she couldn't believe that something that large could be a real diamond. "I thought it was a piece of glass," she said.

Gilbertson named her find – described by Park Interpreter Waymon Cox as "icy white with a metallic luster" – the Illusion Diamond.

Earlier this month, a visitor found a 3.86-carat triangular white diamond that he named the Heart of Arkansas. Since the beginning of the year, park visitors have discovered a total of 196 diamonds.