Witch balls sometimes measure as large as seven inches (18 cm) in diameter. The witch ball is traditionally, but not always, green or blue in colour and made from glass (others, however, are made of wood, grass, or twigs instead of glass). Some are decorated in swirls and brilliant stripes of various colours. Witch balls normally have a hole in the top where a peg can be inserted; string is then attached to the peg so the ball can be hung in a chimney or over a window. Early witch balls often had a short neck sealed by a stopper. The gazing balls found in many of today's gardens are derived from the silvered witch balls that acted as convex mirrors, warding off evil by reflecting it away.