These pieces of magical jewelry can come in almost any design - braided cord with amber; silver links with topaz; whatever. But they always take the form of a necklace with a single valuable stone about the size of a thumbnail. When worn the Necklace of Warning gives its wearer a split second flash of warning about danger thus preventing the wearer from being Surprised, in or out of combat.
Being magical, the necklace does have limitations. It cannot warn its wearer of attacks from a person under the effects of a Mind Bar or someone wearing an Amulet of Undetection. Likewise, a person within an Anti-Magic Shell or under a Scroll of Protection from Magic would not activate the necklace.
If you feel this effect is too powerful, reduce the Necklace to simply reducing Surprise by 1 or 2 rounds.