diff --git a/GROUPS.md b/GROUPS.md
index 00ebd742de..99ea6823a4 100644
--- a/GROUPS.md
+++ b/GROUPS.md
@@ -67,7 +67,8 @@ Please read <http://minecraft.gamepedia.com/Breaking> to learn how digging times
 ### Declarative groups
 These groups are used mostly for informational purposes
 
-* `solid=1`: Solid block (automatically assigned)
+* `solid=1`: Solid full-cube block (automatically assigned)
+* `opaque=1`: Opaque block (automatically assigned)
 * `not_solid=1`: Block is sold (only assign this group for nodes which are automatically detected as “solid” in error
 * `water=1`: Water
 * `lava=1`: Lava
diff --git a/mods/CORE/mcl_autogroup/init.lua b/mods/CORE/mcl_autogroup/init.lua
index 63ddec9e5f..aeea1751a8 100644
--- a/mods/CORE/mcl_autogroup/init.lua
+++ b/mods/CORE/mcl_autogroup/init.lua
@@ -82,6 +82,11 @@ local overwrite = function()
 				newgroups.solid = 1
 				groups_changed = true
 			end
+			-- Automatically assign the “opaque” group for opaque nodes
+			if not (ndef.paramtype == "light" or ndef.sunlight_propagates) then
+				newgroups.opaque = 1
+				groups_changed = true
+			end
 
 			local function calculate_group(hardness, material, diggroup, newgroups, actual_rating, expected_rating)
 				local time, validity_factor