In his notes, Stoker recorded an incident in 1885 when a Russian schooner named Dmitri, travelling from Navra, ran into Whitby harbour. The Whitby Gazette on 24 October 1885 reported that “the Russian schooner Dmitri of Navra, with silver sand, came in suddenly, in heavy weather, but going ashore in “ Collier's Hope” became a total wreck.”
Varna (a real Bulgarian port) is an anagram of Navra, which does not seem to exist. Someone is getting confused.
Silver sand is a soft mortar sand used in building.