These are hex balls or halves that create a ball.

These work great in marker mode tracking for 3d scanners.

Simply print a pair. 
Add marker dots, reflective ones are the best.
Then place one 6mm x 4mm magnet into the whole at the bottom.
You will want each pair to have an opposite side magnet.

One being North pole up while the other South pole up. This will allow them to stick together, while also sticking to metal items.

Ive printed mine in black PETG. This works out very well as the halves themself do not show up in the scans. Making it very easy to remove from the scan later