Last session, the players found a strange vein in the walls of a cavern. They wanted to see if they could break it down or tear chunks out of the wall by targeting it with a spell.
Does a cave wall count as an object for the purpose of being targeted by spells? If so, would an entire cave complex be considered a single object?
DMG 246 implies that a wall is an object:
Use common sense when determining a character's success at damaging an object. Can a fighter cut through a section of a stone wall with a sword? No, the sword is likely to break before the wall does.
DMG 247 confirms that walls are objects, stating that castle walls count as objects:
Big objects such as castle walls often have extra resilience represented by a damage threshold.
Also, Daern's Instant Fortress has parts that are explicitly given HP (DMG 161):
The roof, the door, and the walls each have 100 hit points, immunity to damage from nonmagical weapons excluding siege weapons, and resistance to all other damage.
If the walls of these structures count as objects, then it makes sense that a cave wall would also count as an object for the purposes of spells, since the two things are similar. However, the DMG treats individual sections of walls as separate objects. While walls are objects, entire buildings are not (DMG 246):
For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.
This falls under the "use common sense" clause of the first quote. In the case of the Instant Fortress, reducing one of the walls to 0 HP does not reduce the entire structure to dust, for instance.
Therefore, the DM gets to use their "common sense" to determine what happens when a portion of a wall is targeted. If a wizard casts True Polymorph on that cave wall, I, as a DM, would not rule that the entire cave complex (or the entire planet!) gets polymorphed--only a big chunk of it.
As you can see, the rules explicitly give the DM a lot of latitude on how PCs interact with their environment.
Check this question and answer about What is considered an object.
From DMG p. 246 or here in the DM's basic rules:
For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.
Specifically, a wall is not a discrete item, thus not an object.
In other words the text spells out what is affected. Anything that is not mentioned will not be affected by the spell. A spell that states "every creature in the area ..." and makes no mention of objects will have no effect on objects.
You can stick to the above and when asked for an explanation just wave your hands and say "It's Magic!". However it is not completely unreasonable that, say, a thunderwave spell would break windows, even though they are secured objects and per RAW it would not affect them in any way.
If you decide to deal with this, take a look at the rules on objects on pages 246-7 of the DMG.
You will have to make a ruling on a more or less case by case basis, but generally there are two things to consider: damage type and amount. For example a stone wall would be immune to psychic, poison, lightning, fire, cold, slashing and piercing damage. (Maybe only resistant to both if they alternate fire and cold damage.) It also has a damage threshold of about 10, meaning any damage below that is ignored. (Maybe make an exception for thunder damage.) Finally you have to assign hit points to it, or at least ballpark it. To break a chunk of stone about as large as a doorway off, 30-50 damage will have to be dealt to it, depending on depth.
