There are 3 chain stores that sell jade/jewelry = Chow Tai Fook, Chow Sang Sang & Lukfook, also plenty of independent shops look for the QTS sign (Quality Tourism Service), shops accredited by the Hong Kong Tourism Board.
Quality Tourism Services (QTS) Scheme-accredited Shops Hong Kong Tourism Board
Also there the well known Jade market between Yau Ma Tei & Jordan MTR station.
Jade Market and Jade Street Hong Kong Tourism Board