Mateen made the expensive purchases on June 6 at a Kay Jewelers in Jensen Beach, Florida, which is near the Lotus Gun Shop where he attempted to buy body armor, ABC News reported.
Six days later, 29-year-old Mateen killed 49 people and injured 53 others inside the Pulse nightclub in Orlando.
Mateen - born in the US to Afghan parents - had pledged allegiance to the leader of the Islamic State group during the attack. He was killed during a police raid.
"Signet was shocked and deeply saddened by the violent acts against the LGBT community. We join all Americans in mourning and are working closely with the authorities. We will do everything possible to support them," Signet vice president David A Bouffard said in the statement.