The TriG specification has a Candidate Recommendation  that contains a few minor changes to the previous specification that was being used as the reference for the Rio TriG parser and writer.
The default graph does not need to be enclosed in braces anymore
The syntax has been updated to match the Turtle Candidate Recommendation syntax
Added the GRAPH keyword and the BASE and PREFIX keywords from SPARQL
Graph labels can be blank nodes
Dropped official support for = graph naming operator and no optional "." after each graph