Erinfest/mods/MAPGEN/mcl_geodes/init.lua

local adjacents = {
	vector.new(1,0,0),
	vector.new(-1,0,0),
	vector.new(0,0,1),
	vector.new(0,0,-1),
	vector.new(0,1,0),
	vector.new(0,-1,0)
}

local function set_node_no_bedrock(pos,node)
	local n = minetest.get_node(pos)
	if n.name == "mcl_core:bedrock" then return end
	return minetest.set_node(pos,node)
end

local function makegeode(pos,def,pr)
	local size = pr:next(5,7)
	local p1 = vector.offset(pos,-size,-size,-size)
	local p2 = vector.offset(pos,size,size,size)
	local calcite = {}
	local nn = minetest.find_nodes_in_area(p1,p2,{"group:material_stone"})
	table.sort(nn,function(a, b)
		   return vector.distance(pos, a) < vector.distance(pos, b)
	end)
	if not nn[1] then return end

	for i=1,math.random(#nn) do
		set_node_no_bedrock(nn[i],{name="mcl_amethyst:amethyst_block"})
	end

	for k,v in pairs(minetest.find_nodes_in_area(p1,p2,{"mcl_amethyst:amethyst_block"})) do
		local all_amethyst = true
		for kk,vv in pairs(adjacents) do
			local pp = vector.add(v,vv)
			local an = minetest.get_node(pp)
			if an.name ~= "mcl_amethyst:amethyst_block" then
				if minetest.get_item_group(an.name,"material_stone") > 0 then
					set_node_no_bedrock(pp,{name="mcl_amethyst:calcite"})
					table.insert(calcite,pp)
					if pr:next(1,5) == 1 then
						set_node_no_bedrock(v,{name="mcl_amethyst:budding_amethyst_block"})
					end
					all_amethyst = false
				elseif an.name ~= "mcl_amethyst:amethyst_block" and an.name ~= "air" then
					all_amethyst = false
				end
			end
		end
		if all_amethyst then set_node_no_bedrock(v,{name="air"}) end
	end

	for _,v in pairs(calcite) do
		for _,vv in pairs(minetest.find_nodes_in_area(vector.offset(v,-1,-1,-1),vector.offset(v,1,1,1),{"group:material_stone"})) do
			set_node_no_bedrock(vv,{name="mcl_blackstone:basalt_smooth"})
		end
	end

	for k,v in pairs(minetest.find_nodes_in_area_under_air(p1,p2,{"mcl_amethyst:amethyst_block","mcl_amethyst:budding_amethyst_block"})) do
		local r = pr:next(1,50)
		if r < 10 then
			set_node_no_bedrock(vector.offset(v,0,1,0),{name="mcl_amethyst:amethyst_cluster",param2=1})
		end
	end
	return true
end

mcl_structures.register_structure("geode",{
	place_on = {"mcl_core:stone"},
	noise_params = {
		offset = 0,
		scale = 0.00022,
		spread = {x = 250, y = 250, z = 250},
		seed = 7894353,
		octaves = 3,
		persist = 0.001,
		flags = "absvalue",
	},
	flags = "place_center_x, place_center_z, force_placement",
	y_max = -24,
	y_min = mcl_vars.mg_overworld_min,
	y_offset = function(pr) return pr:next(-4,-2) end,
	place_func = makegeode,
})