The Heavy Obscurred status granted by Fog Cloud and Darkness effect seems to override both True Sight granted by the Gem of Seeing (possibly also the spell granting it) as well as the Shadow Tamer Ranger's Tremorsense.
The 2nd lvl Darkness spell seems to work exactly the same as Fog Cloud, a 1st lvl spell.
Tremorsense also detects flying creatures, which it shouldn't, but not invisible grounded creatures, like it should. How Tremorsense is supposed to work: All Scenes Seismic Sense in avatar - YouTube
D&D 5e rules for Tremorsense and True Sight (from DNDBeyond):
A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance.
Tremorsense can't be used to detect flying or incorporeal creatures. Many burrowing creatures, such as ankhegs, have this special sense.
A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane within the same range.
Typos happen. More so on the phone.