From 'The Bloody Chamber' by Angela Carter.
Carter's description of the necklace is very powerful, conveying many of the central themes of 'The Bloody Chamber'.
She describes the necklace later as "bright as arterial blood", and a "bloody bandage of rubies".
I suggest you read the short story to get a sense of the necklace in context, as the husband's gift to the girl as well as a representation of the husband's lavish (but dangerous and visceral) world. It works so powerfully to convey a sense of horror mingled with luxury and excess. I think the last four words in particular with the huge contrast between the words "extraordinarily precious" and the very visceral image of the "slit throat" emphasise this.
Carter also uses the necklace to convey the girl's potentiality for corruption. Initially, it seems to throw into sharper relief her innocence and youth but later it symbolises both the husband's world and her potential for becoming part of that world.
Anyways, I am studying Bloody Chamber for my English Literature coursework at the moment and I think this just shows Carter's incredible ability to create symbols with about a thousand meanings.