Jadeite is a pyroxene mineral, of ideal composition NaAlSi2O6, but with colouring metal ions replacing the Al. Usually it is green in colour, but can be white, mauve, emerald green etc. It is very tough, hardness 6 1/2 to 7 on Moh's scale, density 3.3, and microcrystalline like a quartzite. It will scratch nephrite jade, just. Roaming around town you will find street stalls offering cheap jade jewellery which is attractive for the tourist. Often it is a jadeite-albite rock having white patches of the feldspar, but looks quite nice. You have to go to lapidary/jewellery shops to see the valuable stuff, and pass the gun-toting armed guard on the way in.
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More details on Gautemalan jade are given in my Bootsnall article Jade at Antigua