Apologies if somebody's already suggested this technique - but
here's what we used to do (about 25 years ago!) in my parents'
little jewelry shop in Cornwall, UK.
We made a tool, which consisted of a length of 4" length of 1/4"
wooden dowel (dimensions not critical ). In each end of this
dowel, drill a small pilot hole, and then insert a domed or pan-head
wood screw - so that only 1/4" of the shank of the screw is clear of
the end of the dowel.
To close a jump ring, you now only need one pair of suitable pliers,
and this magic tool.
Hold one side of the jump ring in the pliers, locate the other side
of the jump ring in the slot in the screw head, and a quick twist
(maybe with a gentle 'push') will close the ring. Because you are
not trying to wrestle with two pairs of pliers, you have much more
control over what's going on.
We must have used our tool to assemble thousands of tumble-polished
gemstone pendants, earrings etc - and it must have cost us all of a
Hope this helps somebody (and thanks for all the help I received
about my 'bearing replacement on lap saw' query some months ago)