These necklaces can be warn but provide no additional properties.
You see an amethyst necklace. It weighs 4.30 oz.
You see a diamond necklace. It weighs 5.00 oz.
You see an emerald necklace. It weighs 4.50 oz.
You see a garnet necklace. It weighs 5.00 oz.
You see a rhodolith necklace. It weighs 5.50 oz.
You see a sapphire necklace. It weighs 6.00 oz.
Sources:
Verified item descriptions on both
https://tibia.fandom.com/wiki/Main_Page