Goldsmith's Hall in the City of London. Why is that of interest? Because once upon a time, if you had a bit of gold or silver that you wanted to sell, you had to take it to the Goldsmith's Guild where they would test its purity. They would then stamp it with the mark of the Goldsmith's Hall, which guaranteed its value to a buyer.
In the racy classic An Exact Abridgement of All the Statutes of King William and Queen Mary (1700) it says:
Any Persons, Natives or Foreigners, who by themselves or others shall bring any sort of Wrought Plate between 1 Jan 1696. and 4 November, 1697. to any of his Majesty's Mints, or to such Persons as shall be Authorized to Recieve the same, shall be then and there paid for such Plate at 5 s. 4 d. per ex. And all such Plate, having the Goldsmiths Hall Mark and Workman's Mark, shall be Received as Sterling Silver*
And that, my child, is where the word hallmark comes from. It's the mark of quality given out in Goldsmith's Hall. It's also why the building in the picture is the origin of all those greetings cards and the American TV channel.
Well, in fact, that building wasn't erected until the 1820s. In William and Mary's time it looked like the picture below.
By 1809 the word was being used figuratively to mean a characteristic stamp. So a sermon could have the hallmark of orthodoxy. Thus the cards.
*The OED doesn't have a citation for hallmark until 1721, and nothing for the figurative use until 1864. Do I get a prize?