I read it when I was five or six in about 1997? From what I remember, in the story a child discovers a lake or pool in the mountains that shimmers with beautiful light from the stars because it's lined with diamonds or crystals. The child keeps it secret from the grown ups because they're greedy and will ruin it for their own gain. At the end the adults eventually force the child to show them the pool, or discover it by following them (I can't remember) and the sun comes up when they arrive which blinds them all because of the light reflecting from the bottom.
It may have been published in the 90's, it may be from before that. My primary school got it around the same time as 'Thud!' by Nick Butterworth