Shares of Bermuda Commercial Bank Ltd plunged 13.7 percent yesterday on low volume in Bermuda Stock Exchange trading.The bank’s shares fell $1.70 to $10.75 on the trading of 100 shares.The only other domestic-board stock to trade was Butterfield bank, which held firm on $1.30 as 2,500 shares changed hands.The Royal Gazette /BSX Index fell 10.2 points, or 0.9 percent, to close on 1,119.Volumes was 2,600 shares and turnover was $4,325.The BSX Insurance Index of the Island’s leading international insurance companies gained 31 points, or 3.24 percent, to close on 987 points.