"I’ll take that one," she says, pointing, and at the same time picks up some light paper, a box of soft pencils and a hard steel clipboard debossed with the head of Oliver Cromwell on a spike. The sales assistant is very keen to sell her a lucite one instead, but the Mohs scale of hardness awards acrylic glass no more than a four, as against steel’s five or six, so the Inspector demurs, wishing the shop’s inventory stretched to products in tungsten carbide.