One sphere was available in a repaired state and required some fill where it had been glued together. It couldn't be taken apart without damaging the bone further. I felt that if filled, it could continue to be used and would be an example to copy.
I first turned my thoughts to wooden balls sold as knobs or door pulls to make a second sphere. They weren't very convincing, and I dreaded what they would look like surface-painted.
My collection of Fimo polymer clay in a Ziploc bag was getting quite crumbly and difficult to work with; and I was beginning to wonder how I could proceed without investing in a lot of new materials.
Happily, there are many YouTube videos that show how to reconstitute the clay, as well as how to blend and do colour effects. Many of these videos give tours of workspaces and are full of tips, tricks and advice. I fell down the rabbit hole of the polymer clay crafting world.
The first thing I learned was that Fimo is not the polymer clay of choice amongst crafters. However, I seemed to have the right colours in my Ziploc. Buying both a liquid polymer clay and a block of 'softening' polymer clay transformed the Fimo I had into usable, blendable material.
After mixing some colours, I sliced a ball of my mix into many slices and dusted the inside surfaces with ground raw umber pigment, smacked it together again and tried to round it back into a sphere without disturbing the dark bands too much.
Some wire nails were cut shorter to press into the sides to imitate the silver inclusions on the original finial. Here they are matching pretty well. The new bottom one is not yet totally round.
Unfortunately, the baking darkened my new finial quite a bit. Also, it had to get a lot rounder.
Here it is going into the oven.
The baked sphere was easier to sand to roundness. A touch of white 'pickling' polyurethane mixture helped lighten it a bit, and a rub of polyurethane sealer gave it the same sheen as the bone original.
Here is the finished effect.
Here they are perched on top of their post supports with museum wax. A silversmith will have to straighten the posts and reinstall the finials after that is finished.