I'm not sure if this is the intent, but the Ring of Earth Elemental Command states (DMG pp 190)
You can move in difficult terrain that is composed of rubble, rocks, or dirt as if it were normal terrain.
It also states:
You can move through solid earth or rock as if those areas were difficult terrain
The implication here in RAW is the following:
- Ring grants passage through difficult terrain made of rock/dirt/rubble at normal pace
- Ring grants passage through solid earth/rock as difficult terrain
- Solid earth/rock is composed of rocks/dirt/rubble
- Therefore, it becomes "difficult terrain that is composed of rubble, rocks or dirt"
- This then activates the passage through difficult terrain made of rocks/dirt/rubble at normal pace for going through solid earth/rock.