Necklaces come in various gemstones. Metal is nickel free, tried and tested by Miss Boo herself, who suffers with an allergy to the evil product:
 
Carnelian
Clear Quartz
Unakite
Rose Quartz
Amazonite
Lapis Lazuli
Citrine
Garnet
Turquoise 
Lepidolite
Amethyst
Tourmaline
Rhodenite
Jade
Howlite
Orange Citrine
Tigers Eye
Synthetic Goldstone
 
Helps to ward off evil spirits and bad juju... probably...