diff --git a/GROUPS.md b/GROUPS.md
index 688441d0ae..cdfd2c07b6 100644
--- a/GROUPS.md
+++ b/GROUPS.md
@@ -95,6 +95,7 @@ These groups are used mostly for informational purposes
 * `solid=1`: Solid full-cube block (automatically assigned)
 * `opaque=1`: Opaque block (automatically assigned)
 * `not_solid=1`: Block is not solid (only assign this group for nodes which are automatically detected as “solid” in error
+* `not_opaque=1`: Block is not opaque (only assign this group for nodes which are automatically detected as “opaque” in error
 * `fire=1`: Fire
 * `water=1`: Water
 * `lava=1`: Lava
diff --git a/mods/CORE/mcl_autogroup/init.lua b/mods/CORE/mcl_autogroup/init.lua
index f6d0e259b4..78067c803c 100644
--- a/mods/CORE/mcl_autogroup/init.lua
+++ b/mods/CORE/mcl_autogroup/init.lua
@@ -82,7 +82,8 @@ local overwrite = function()
 				groups_changed = true
 			end
 			-- Automatically assign the “opaque” group for opaque nodes
-			if not (ndef.paramtype == "light" or ndef.sunlight_propagates) then
+			if (not (ndef.paramtype == "light" or ndef.sunlight_propagates)) and
+					(ndef.groups.not_opaque == 0 or ndef.groups.not_opaque == nil) then
 				newgroups.opaque = 1
 				groups_changed = true
 			end
diff --git a/mods/ITEMS/REDSTONE/mcl_observers/init.lua b/mods/ITEMS/REDSTONE/mcl_observers/init.lua
index 825a8c9a44..ece8e39c58 100644
--- a/mods/ITEMS/REDSTONE/mcl_observers/init.lua
+++ b/mods/ITEMS/REDSTONE/mcl_observers/init.lua
@@ -80,7 +80,7 @@ mesecon.register_node("mcl_observers:observer",
 	sounds = mcl_sounds.node_sound_stone_defaults(),
 	paramtype2 = "facedir",
 	-- TODO: Add to craft guide and creative inventory when it's useful
-	groups = {pickaxey=1, material_stone=1, not_in_craft_guide=1, not_in_creative_inventory=1 },
+	groups = {pickaxey=1, material_stone=1, not_opaque=1, not_in_craft_guide=1, not_in_creative_inventory=1 },
 	on_rotate = false,
 	_mcl_blast_resistance = 17.5,
 	_mcl_hardness = 3.5,
@@ -133,7 +133,7 @@ mesecon.register_node("mcl_observers:observer_down",
 {
 	is_ground_content = false,
 	sounds = mcl_sounds.node_sound_stone_defaults(),
-	groups = {pickaxey=1, material_stone=1, not_in_creative_inventory=1 },
+	groups = {pickaxey=1, material_stone=1, not_opaque=1, not_in_creative_inventory=1 },
 	on_rotate = false,
 	_mcl_blast_resistance = 17.5,
 	_mcl_hardness = 3.5,
@@ -181,7 +181,7 @@ mesecon.register_node("mcl_observers:observer_up",
 {
 	is_ground_content = false,
 	sounds = mcl_sounds.node_sound_stone_defaults(),
-	groups = {pickaxey=1, material_stone=1, not_in_creative_inventory=1 },
+	groups = {pickaxey=1, material_stone=1, not_opaque=1, not_in_creative_inventory=1 },
 	on_rotate = false,
 	_mcl_blast_resistance = 17.5,
 	_mcl_hardness = 3.5,