Marcasites are a pain, IMHO. They take no heat, are oftentimes glued
in, and replacements are hit or miss as far as match goes.
If you’re lucky the stones are confined to the top and (really)
hopefully the join will be far away enough that you can isolate the
If you have some room between the stones and new join you can
suspend the ring top upside down in water. You’ll need a lot of heat
and be quick about it. Use easy or extra easy solder.
If silver soldering just won’t work, you can make a new shank with
what we used to call a dishpan. A larger inverted semi dome that fits
to the underbezel. And soft solder it. Not purist but then sometimes
you don’t get the luxury.
Once the join has been made watch out for the stones to fall out
during cleaning. Or subsequent wear.