When you craft mese crystal seed and place it on obsidian surrounded by lava the crystal will grow.
Use Crystalline bell to cut pieces of grown crystal without destroying it.
Have fun :)
Repo: https://github.com/minetest-technic/mese_crystals
Download zip (as usual rename the folder to "mese_crystals" ): https://github.com/minetest-technic/mes ... master.zip
Crafting Crystalline Bell:

Crafting Mese Crystal Seed:

Screenshots:
Crystals underground:

Crystals farm:

License: WTFPL