Police say the pair, pretending to be customers, were shown a number of pieces on Tuesday.
"It's alleged that, while no one was looking, the man entered a workshop area and stole a number of rare high-quality diamonds," police said in a statement.
The couple left the shop a short time later while the theft was noticed only the following day.
Police say the man was about 40-45 years old. He was wearing a short-sleeved checked shirt, dark blue denim jeans, dark grey casual shoes and dark sunglasses.
The woman was about 35-40, about 155-160 centimetres tall, with a medium build and dark olive skin. She was wearing a pink T-shirt with "I love Australia" written on it, dark blue jeans, dark shoes, a tan coloured bucket hat and was carrying a large bag.