If you had to rank the world’s most powerful cities, Hong Kong would be a serious contender for No. 1. Like New York, it is a hub of money, the financial engine that makes Asia go round. Like Mumbai or Mexico City, it teems with humanity. Like London, it is home to architecture that reflects the colonial influence of Old Britannia. In Hong Kong, power is both old and new, and utterly palpable.I was reminded of all these things in early March, when I attended the 31st edition of the Hong Kong International Jewellery Show, the largest spring ­jewelry platform on the planet. It was there that