 In the resulting discussions, Terrell suggested replacing the cork with rock to deflect the hard core of an armour-piercing bullet so that it hit the backing plate at an angle, dispersing its energy so that it would not penetrate.Terrell recalled an old legal case that he had conducted for the Amalgamated Road Stone Corporation.While studying background technicalities, he had visited Penlee Quarry at Newlyn in Cornwall where he was told that Penlee granite was the hardest available.