Hong Kong stocks rallied Friday, ending a volatile month on a high, after Japan announced a surprise negative interest rate policy, effectively charging banks to store their cash in a bid to kick-start lending in the country. The Hang Seng Index rose 2.54 percent, or 487.28 points, to close at 19,683.11. However, the index still ended the month down about 10 percent, swiped by concerns over the global economy, particularly China, and plunging oil prices.