The necklace was created by Louise Truman at the Navajo reservation after she gave birth to her daughter Patty. The necklace went with Patty when she was adopted. It has since passed down the DeLuca family, from Patty to her daughter Mimi, who later gave it to Maria as a high school graduation present.
The necklace contains a flower that inhibits alien powers. According to Mimi, it will keep Maria "safe from evil", presumably preventing her from developing the genetic degenerative brain disorder which is presumably caused from using her powers.
Maria wears the necklace given to her by her mother. When she and Michael Guerin hook up in Texas, she thinks she's lost it but it was caught in Michael's boot. The clasp is broken, so Michael fixes the clasp and returns it to her.
When Liz Ortecho sees the necklace, she recognizes that the flower in the necklace is the same rare flower that releases a pollen that paralyzes alien abilities.
After Mimi DeLuca goes missing and is found, Maria takes the necklace off and puts it on her mother.
Her mother later insists that Maria wear it, saying it will protect her from evil. Michael realizes that the flower is the same one that paralyzes alien powers, and that it might limit her powers. Maria decides not to wear the necklace while she learns about her own powers, and she lets Rosa Ortecho wear it instead while she's attending rehab.
The necklace is Navajo and has the word "Tsela" on the back. Looking for more information about it, Maria accompanies Alex Manes and the others to the Navajo reservation. Alex says that "Tsela" means "star on the ground". Isobel and Max recognize this term because it's what Isobel's mother Louise called her two daughters. Isobel asks when Maria's grandmother was born, and they realize that Louise's second daughter is Maria's grandmother. Louise left the necklace with her daughter when she was adopted.