Earth Glide. The elemental can burrow through nonmagical, unworked earth and stone. While doing so, the elemental doesn't disturb the material it moves through.

Siege Monster. The elemental deals double damage to objects and structures

Innate Spellcasting. The elemental's innate spellcasting ability is Constitution (spell save DC 17, +9 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: magic stone, meld into stone

1/day each: wall of stone, flesh to stone

Magic Resistance. The elemental has advantage on saving throws against spells and other magical effects. 

Multiattack. The elemental makes three slam attacks. It can replace on of the slam attacks with a boulder attack.

Slam. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Boulder. Ranged Weapon Attack: +10 to hit, range 1100/400 ft., one target. Hit: 17 (2d10 + 6) bludgeoning damage.