Vermiculation is not a stone but a form of rustication ie an exaggerated treatment of masonry to give the appearance of strength.
With vermiculation this is achieved through a styalized texture mimicking worm casts. True vermiculation seems be be rare in Canterbury – the Old Prison example below is referred to as 'vermiculated' in the Listed Buildings record, but in honesty looks little like worm casts. The Whitstable Road and Bridge keytones provide more convincing examples.
Local examples (click to enlarge)
Vermiculated (or similar) surround to the doorway of Canterbury Prison (closed as a prison end March 2013)
Vermiculated keystone above a window – one of a set – on Bridge Hill House near Bridge south of Canterbury
Vermiculated keystone above a window – one of a set – on Whitstable Road