Not using them currently but I have some 3mm cubic zirconia. They allow lower temps, but are also more restrictive. A 3mm gem is probably half the size of a sphere so I've wondered how the larger ones would perform.
Gem cut stones will sit closer together reducing airspace, and the points and hard edges are capable of scratching glass and quartz glass.
The edges are also more prone to chipping, even diamonds are not indestructible and can chip.
That being said after dropping my qaroma and not realising there was any damage i used it with tiny loose chips of quartz in it for some time with no ill effects. ill effects .lil effects. ell ifficyts. fill efix.
The qaroma took a lickin and kept on tickin tho.
If you want cheap gems, tumble polish them first to buff the edges off, but spheres are a better shape for the job, and ruby is thermally superior to zirconia if you look at the specs.
Ruby Heats faster, has more heat capacity, passes more heat on, has a quicker recovery time.
Everyone wants to believe zirconia is better cause its cheaper. And comes in more colours. It is thermaly almost identical to quartz glass, and not so different from borosilicate glass. Just tougher.
Wish it were not so because zirc is cheaper and comes in any colour you like.
Some may disagree with this. Some may think rubies and borosilicate taste the same.
In the end if you like the taste of zirconia more thats all that matters. But i would go with balls over gems.