I've used my Speedbrite ionic cleaner many times on turquoise and
silver pieces. And it works great no damage to the stones. Both
natural and stabilized turquoise is safe to clean with the ionic. I'm
not sure of the physics at work but an ionic cleaner is different
than an ultrasound. An ultrasound can pummel turquoise to bits if you
are not careful.
The one thing you might be careful is that if the turquoise is set
the traditional Native American way with saw dust between the stone
and bezel, if the sawdust gets wet it can swell and loosen the stone.
But the limited time it will be in the ionic cleaner there's not much
chance of that.
Silversmith and Lapidary Artisan
Colorado Springs, Colorado