The simple answer is yes, when:
1. Made by a responsible company who has lab tested their designs
2. Used correctly, following the safety information provided.
In recent years, there has been an explosion of companies and individuals selling teething jewellery. As with anything designed to go in a baby's mouth, the top priority has unquestionably got to be safety.
Different countries have different safety regulations for products and in the UK we follow European safety standards. For teething jewellery (and indeed all jewellery) there are no specific safety standards that businesses have to legally comply with, other than they must not contain nickel, lead and cadmium. But every seller has a responsibility to ensure that they are providing a "safe" product, the definition of which is open to interpretation.
After many months of research the only way that I felt comfortable that we were providing safe teething jewellery was to have my designs lab tested. Now this is easier said than done as what do we test them for if there are no set requirements to test against? With advice from UKAS accredited labs, I decided to have my products tested against the relevant parts of EN71, parts 1, 2 & 3 (these are the toy safety regulations), and a General Product Safety Assessment, which importantly includes a 'bite test'. The jewellery passed all tests, other than the really tiny 9mm beads failed the bite test. I don't use these beads for anything to be chewed but many other do businesses use these beads.
Testing is very expensive and time consuming (even to purchase a copy of the safety standards is hundreds of pounds) so if you see very cheap products for sale, in all likelihood they will not have been safety tested.
So in summary, when you buy a teething necklace, please make sure you read the product and company information to make sure they have lab tested their jewellery in a UKAS accredited lab. The other important point is that teething necklaces are not safe to be handed to a baby. Read the safety information that is provided and follow it strictly. Teething necklaces are not a toy and should never be treated as such.
A word about amber teething jewellery...
Some people swear by the natural soothing properties of amber but amber teething products are not something that will ever be sold by Seb & Roo. In my view, amber jewellery is unsafe and I have read reports of baby deaths and seen numerous product recalls for amber jewellery. Unlike silicone teething jewellery, amber jewellery is designed to be worn by the baby and poses a significant risk of choking and strangulation. If you do use them, please never let your child sleep in them or put them anywhere near their mouth.