I haven't formerly trained/qualified in hot stones or cold stone massage. However, I do have a license in Esthetics (600 hours) and adjoining level 4 qualifications for advanced facials here in the UK.
I don't follow the holistic/alternative medicine side of using this crystal for facials. I've used rose quartz rollers and sweepers/gua shua tools in facials while working in spas here in the UK. I imagine you wouldn't need training/certificates to offer this,certainly if you have credentials to offer facial massage.
I love this as a tool for the fact that it's cooling. This is great for stimulated/sensitive skin types that are easily reddened especially after facial manipulations/massage/peels or enzyme treatments. I find this to be such a lovely add on to gently cool the surface of the skin while also using it to perform some lymph drainage techniques. Very popular and certainly adds to the overall sensorial experience of a facial, warm, cool, relaxing aromaras, touch.
As far as material is concerned, I prefer stainless steel facial tools as they are non porous making them more hygienic which is crucial to me.