Theroux, 41, proposed to Aniston on his last birthday, reported Showbiz spy.
"Justin is planning to buy Jennifer a massive ruby because it symbolises devotion, integrity, courage and happiness and he wanted his gift to symbolise what he is offering her as her new husband. Once he finds the right stone, he will have it made into a necklace," a source said.
Aniston was said to be impressed with the eight carat diamond engagement ring that Theroux gifted her.
"It's a huge emerald-cut diamond. It's a rectangle with tapered diamond baguettes on the sides. It's beautiful. Just after Jen got engaged, she called all her friends and told them about it. She was so happy and said she loves the ring. I think she was overwhelmed and overjoyed by the size and beauty of it!," the source added.
