This robin's-egg-blue, opaque, nearly inclusion-less stone caused me some hesitation. My most confident assessment is that this is a dyed turquoise agate, although honorable mentions of possibilities include amazonite or true turquoise. Regardless, it is absolutely gorgeous.
This piece is truly costume/fashion jewelry being unfortunately not guaranteed hypoallergenic. Stone setting is a tin metal alloy. Necklace is 20" combined of stainless steel chain, separated into two strands for the design of this necklace.