Answer:
Lovely and round, I shine with pale light, grown in the darkness, a lady's delight.
A little pool with two layers of wall around it. One white and soft and the other dark and hard. Amidst a light brown grassy lawn with an outline of a green grass.
Break it and it is better, immediately set and harder to break again.
It's been around for millions of years, but is never more than a month old. What is it?
What goes up but never comes down?
What is always coming but never arrives?