Matthew Smith of Dallas, Texas, recently found his “brown-eyed beauty.”

In his first visit to the Crater of Diamonds State Park near Murfreesboro, Ark., Smith, who is only 9, found a 2.75-carat brown diamond described as being the size of an M&M candy.

He found the gem about 3:30 p.m. July 12 while searching the ground by a path near the Beatty’s Hill portion of the park’s 37-acre search area.

The diamond was further described as being a dark, warm brown—about the color of a cola drink. Matthew named his stone the “Brown-Eyed Beauty” after the color of his ...