The lockets were created by Max Hamilton and were given to the 3 original mermaids. Each locket has a different colored stone. They are the same in many ways, but they are different, too. Each gemstone represents the girls' powers.
Emma's Locket originally belonged to Louise Chatham in the 1950's. She gives her locket to Emma since she believes that all 3 of them should be together, since Cleo and Rikki already had theirs. The white gemstone which could possibly be a diamond, and represents Emma's ability to freeze water and generate ice and snow at will.
Emma found Cleo's locket at the bottom of the Moon Pool in "Party Girls". After Miriam almost stole it, Emma gave it to Cleo saying "it would be safer with her." In the two-part episode "The Gracie Code" Max tells Lewis he made the locket for Gracie, the love of his life, but after being reminded that Gracie can never be human again, she threw it into the Moon Pool where Emma found it fifty years later. Cleo's locket has a blue stone - presumably a sapphire. Blue for water, the shaping and molding of it. It originally belonged to fifteen-year-old Gracie Watsford, Charlotte's maternal grandmother in the 1950s.
While on a date and shopping with Zane, Rikki spotted the locket in the window of an antique jewelery shop. Zane then asked Miriam (Zane's ex-girlfriend) if girls liked getting jewelery and Miriam spitefully bought the locket, to make Rikki jealous. Rikki tried to get the locket back but lost. Zane then made a deal with Miriam, that if he kissed her she would part with the locket, but Miriam then threw it into the harbor where Emma and Zane both jumped into the water with Emma nearly being exposed. Emma hid behind a post and let Zane search for it instead, and he found it after two tries. Miss Chatham said that Rikki deserves it because she kept the secret from Zane. Rikki's locket has a red stone - which is a ruby. Red for heat, for fire. It originally belonged to fifteen-year-old Julia Dove in the 1950s.