"Um," Harry said. "Actually I was thinking that once I know how, I could Transfigure the rock into a ring and wear it on my finger. If you could teach me how to sustain a Transfiguration -"

"It is good that you asked me first," Professor McGonagall said, her face growing a bit stern. "If you lost control of the Transfiguration the reversal would cut off your finger and probably rip your hand in half. And at your age, even a ring is too large a target for you to sustain indefinitely without it being a serious drain on your magic. But I can have a ring forged for you with a setting for a jewel, a small jewel, in contact with your skin, and you can practice sustaining a safe subject, like a marshmallow. When you have kept it up successfully, even in your sleep, for a full month, I will allow you to Transfigure, ah, your father's rock..." Professor McGonagall's voice trailed off.

So, if it is a training method, it's one McGonagall knows about and in fact specifically suggested.