Gemstone/S was first an object database, per above article. Vaguely reminds me of:

https://en.wikipedia.org/wiki/InterSystems_Cach%C3%A9

Which I heard about from a friend who was using it at work.

https://en.wikipedia.org/wiki/Comparison_of_object_database_...

Caché is first one in the table.

I remember object or object relational databases were popular at one time. maybe they still are to some extent.