They pulled him on to the bank and turned him over. I know this man, he said. His friend said, who owns him? Nobody owns him, but he curses in German, therefore let us take him to the German House, for I myself am staying not at the Tuscan House but with a man who runs a foundry. Karl Heinz said, you are dealing in arms? and he said, no, altar cloths. Karl Heinz said, you are as likely to shit rubies as learn an Englishman’s secrets.
Bring Up the Bodies by Hilary Mantel
What is the meaning of 'to shit rubies' here?