Is there any risk to the eye in gazing at reflected sunlight through a surface such as stone, or glass?

Your normal protective mechanisms which ordinarily prevent you from staring at the sun or a bright light, are still operative when viewing the reflection of the sun off a rock or glass. If the glass is a mirror, then you get the full impact and will avoid gazing at it. I can imagine rocks with mica (a shiny, reflective mineral found in some rocks) which might cast a uniform reflection similar to that from a mirror but your same protective reflexes would be operative. The bottom line is that unless you voluntarily stare at such a bright image, you will suffer no harm.