I’ve driven up there a few times. You can see a looooong way on a clear day.
So have I. I used to live in Clayton and had a view of Mt. Diablo in my living room. Mount Tamalpais is cool too.
In early California land grants Mt. Diablo was used as a marker to help denote borders of property.
I have a BLM map from 1989 that still denotes the Mt. Diablo line, going east to west.
I have a rich cousin who lives near the base of Mt. Diablo. Vendome probably knows him.