Tyrrell, 48, made his find Sunday afternoon in soil that had come from the top layer of dirt where he dug.
"When I'm here and I'm walking, I'm always looking. When I'm on the ground, I'm always digging," said Tyrrell, who says he's found 131 diamonds in his many digs. He said he's been living in a tent at a local rock shop for the past nine months and works as a handyman.
"I do various jobs. It's a slow place, not too many things happen," which he said gives him a lot of time to spend at the mine.
"I come as often as I possibly can out here, depending on the weather and other things that I have to do back at camp," he said.
The light-brown diamond has a rectangular shape, about the size of a piece of Chicklets chewing gum, he said.
"A beautiful gem like that ... very clear. When a diamond shows up, it's beautiful. It's an amazing thing to know, this little spot produces diamonds. I love it a lot," Tyrrell said.
Tyrrell said he'd listen to anyone who wants to buy the gem, but he noted that park rules don't allow him to offer the diamond for sale.
The site, which opened as a state park in 1972, is the world's only diamond-producing site open to the public. Visitors can keep the gems they unearth. The largest diamond found at the state park was the 16.37-carat Amarillo Starlight, a white diamond found by a Texas visitor in 1975.
Garland said the last time 1,000 or more diamonds were found at the park in one year was 1994, with 1,421 diamonds found. The mark was reached in 1990 and six times in the 1980s for a total of nine times since the site became a park.
Tyrrell said that when he found the diamond he first cleaned it, then put it in his mouth, "under my tongue." Then he headed for the visitors center.
"They always know when I'm coming with something," he said. "It was a very exciting day."