The Mint could not find a suitable metal which was sufficiently different in colour to the existing coins and which would not tarnish. This last point was thought to be important because the new coin would be the most valuable coin in general circulation in the world (about £8.40 in today's values). It therefore had to be a different shape; various methods had been used overseas to overcome this problem but none were without drawbacks. A hole through the coin did unacceptable things to the Queen's head (a legal requirement on British coins), and wavy-edged, flat-edged or square coins could not be used in the coin-handling machinery which was then coming into increasing use in industry, banking and vending. To be used in a vending or sorting machine a coin would have to roll under gravity and be capable of being measured without being presented in a special way, in other words it needed a constant breadth at whichever angle it was measured.