Elor finds Nyda, and explains to her the following.
"I'm not a priest, so what I know about it is limited. But what I do know is this: Selune once had a child, which died. I don't remember if it died of natural causes or if it was murdered by Shar. After this, Selune cried, sending her tears down to crash on Toril, impedding themselves into the world as the crystals you know them as now."