How do I find Rock Crystals in Minecraft?

I've been playing Minecraft for a few days with the mod Astral Sorcery (in 1.10.2) but I don't know how to get the Rock Crystals and that is gradually spoiling my game fun.

I think it was under the bedrock

You craft a resonating wall and hold it in your hand at night, then you can discover rays of light. There you dig your way down to Bedrock, there you will find 1-4 Rock Crystals.

https://minecraftguides.net/AS/getting-started/resonating-wand