The condition for the second effect of Earth Tremor reads (emphasis mine):
If the ground in that area is loose earth or stone
Loose earth is adequately explained in this question referencing the Mold Earth cantrip.
My question is: does "loose" apply to both earth and stone, or does Earth Tremor's second effect work on non-loose stone, such as a solid stone floor?
The text is ambiguous, so I'm applying some logic here. If the spell requires loose earth to make it difficult terrain, then, by definition, compact earth would not become difficult terrain.
Since stone is even harder than earth, I'd say the spell text means
If the ground in that area is loose earth or loose stone
And it only becomes difficult terrain if the terrain can actually be shaken and disturbed.
