add default:mese_ongen and use this for the mapgen as replacement for default:mese

mods/default/mapgen.lua

@@ -463,7 +463,7 @@ function default.register_ores()
 
 	minetest.register_ore({
 		ore_type       = "scatter",
-		ore            = "default:mese",
+		ore            = "default:mese_ongen",
 		wherein        = "default:stone",
 		clust_scarcity = 36 * 36 * 36,
 		clust_num_ores = 3,
@@ -474,7 +474,7 @@ function default.register_ores()
 
 	minetest.register_ore({
 		ore_type       = "scatter",
-		ore            = "default:mese",
+		ore            = "default:mese_ongen",
 		wherein        = "default:stone",
 		clust_scarcity = 36 * 36 * 36,
 		clust_num_ores = 3,

mods/default/nodes.lua

@@ -1093,6 +1093,15 @@ minetest.register_node("default:mese", {
 	light_source = 3,
 })
 
+minetest.register_node("default:mese_ongen", {
+	description = "Mese Block",
+	tiles = {"default_mese_block.png"},
+	paramtype = "light",
+	groups = {cracky = 1, level = 2},
+    drop = "default:mese",
+	sounds = default.node_sound_stone_defaults(),
+	light_source = 3,
+})
 
 minetest.register_node("default:stone_with_gold", {
 	description = "Gold Ore",