Spherical Diamond is a trading item in 80 Days. It is acquired in Reykjavik with the right conversation choices, if you have previously met Otto Lidenbrock and told him to try Iceland.
It is quite valuable in places that you cannot get back to, such as New York or Lisbon. If you take the Silver Seal to London, it can be used when talking to the captain to purchase the Silver Seal and attempt speed up the trip If you still have it in London, the butler in the Reform Club will place it over the bar (if he doesn't like one of your other possessions better).