What types of cover are indestructible, or require more than one action to destroy?

Question

I've noticed that some types of cover in the game are completely indestructible. One example are cars. While you can blow a car up, it won't actually destroy it as a piece of cover that you or enemies can hide behind.

I've also noticed that some types of trees actually require more than one action to completely destroy as cover. An explosive will reduce the tree down to a stump (which still counts as high cover), while a second explosive will finally destroy it completely.

Are there any other types of unusual cover in the game that are effectively indestructible, or that require special actions to destroy (like attacking it more than once)?

Answer 1

Actually "full" trees are destructible, and have 3 stages before getting completely destroyed.

1: unharmed
2: 1st hit - full cover trunk, full tree size
3: 2nd hit - full cover trunk, human (or 1 story/floor) size
4: grounded - no cover

Answer 2

There are some alien structures in city center type maps that don't seem to care if I hit them with grenades. Not only do they get not destroyed or damaged, they block the area of effect of explosives. Much to my chagrin when a muton taking cover behind one took no damage from a blaster bomb. The kind of elongated black inscribed blocks are one. There was another alien pillar thing as well but I don't remember what it looked like well enough to describe one.