J

• Jade

  A precious green stone from which jewellery and ornamental stones are made, particularly in China and Japan.

• Jewels

  A precious stone (e.g. sapphire, ruby) which acts as a bearing for gears in a mechanical watch.  The jewels reduce friction to make the watch more accurate and longer lasting.  A quality hand wound or automatic mechanical watch contains at least 17 jewels.

• Jumping Hour

  The name 'Jumping Hour' refers to a principle of time indication in numerals appearing in a subdial. Dates printed on a disc jump from one unit ahead every 60 minutes (at every rotation of the minutes hand). The mechanism to initiate the jump of hours is highly power consuming therefore often a power reserve indicator is necessitated.