Earth Elemental does Earth Glide feature burrow through other materials?

@JeremyECrawford Does the Earth Elemental only have the ability to burrow through its Earth Glide feature so it can't burrow through other materials?

— Dakota Hansen (@DakotaHansen13) January 23, 2018

The earth elemental's burrowing speed follows the normal rules for burrowing (see MM, 8). Earth Glide gives the elemental's burrowing speed some extra spice: the ability to move through unworked earth and stone without disturbing it. #DnD https://t.co/BzXcw3QTjv

— Jeremy Crawford (@JeremyECrawford) January 23, 2018