Kind of an uphill drift to get to Colorado from any coast ! Suspect cross grain pen would look best . Take some more photos when you have a piece cut off the board .
Unfortunately I will not actually get a piece of it - he just wants a pen from this type of wood. I'm guessing that even if I had a piece of driftwood once I turned it down it wouldn't look like that. I may have to resort to treating it after turning down to size.
Turn most any wood down to size, use some coarse grit sandpaper to give it texture then stain it gray. The only way for it to be natural is to leave it out in the weather about 3 to 4 years after turning it.
I remember many years ago when grayed cedar and cypress siding was all the rage in Louisiana. There was a stain/finish that would accelerate the graying of those woods. Might be an option.
See if you can find a piece of weathered cedar fencing. Around here, cedar fences were all the rage about 40 years ago. They turned that color after a while, and being cedar, they didn't rot.