you talking about adding large rocks or more gravel??
Lava rocks would blend in nice (not those tufa type of lava rocks)
Zebra stones would blend in. Though one would want to make some contrast if they are using larger stones for the scape
Flagstone is a style of rock more than a type of rock.
Yours does appear to be a kind of slate.
Huh so an interest in geology does come in handy...... Btw that tank is amazing. Simple to the point and absolutley beautiful. Kind like my girlfriend.