All 'inert' materials including; glass, ceramic, stone, aluminum & steel are safe when 'used as directed'. My wife loves the glass (many), steel (Pure Wand) and aluminum (Alumina Flow) toys we have - clean up is also a snap and temperature play is also very nice - a cool toy on hot skin can have interesting results
Silicone is generally considered the safest 'softer' material out there. Some are softer than others. For example one of our fav's the Fun Factory Tiger - is quite firm - but the texture and size make it perfect for g-spot stimulation. The O2 Revolution on the other hand is a dual density toy with a firm inner structure but a very comfortable soft silicone outer layer which is more 'life like'. My wife loves these as well.
You get the idea that variety is very important - at least my wife loves different sensations - depending on her mood. Imagine that!?
Like many we started out with jelly toys - because they are cheaper. Both my wife and I developed a sensitivity to that material (it causes a severe burning sensation) so we've thrown them all out but 1 - which we must use a condom on.
Many dual stimulators are made with TPR - we have had no issues using TPR. Her fav ones are the Decadent Indulgence and Ultimate Decadent Indulgence. The original DI is much stronger (4 AAs) the newer UDI has just 3 AAs, is weaker - but waterproof. Again it's about the mood!