3rd Newton's cradle. Steel balls, attached by epoxied on nuts.
The balls were colorized by heating with propane torch. Not to red hot, this would leave an ugly scale. With heat they first turn a straw light brown, then finally blue black. I tried to get a range of colors. I first realized I could color polished steel like this while annealing bearings so they could be drilled. They're beautiful in the sunlight. The blueish black is actually from a thin layer of ferrite, an oxide that forms at higher temperature conditions than normal rust.
I made a rod+plate frame for this, possibly a quite useful thing for other people's projects. If you have a lathe and can face rod perpendicular to axis, then drill and tap axially t's very easy to make strong, well aligned frames.
In fact you can also do this with regular pipe or tube and allthread. We've used sched 40 PVC + allthread and plywood to make nice cable dispensers.