Good eyes! The stone appears to be a river rounded cobble. I wondered initially about the indentation on the left image (the one with what might be a fossil). On closer examination, the raised line is more resistant than the rest of the cobble. The cobble may be a well-cemented sandstone. Sandstone doesn't usually wear well under erosive conditions.
It doesn't appear to have been worked, as in human shaped.
King County, WA has a rich geologic history. In recent times (the last 12,000 years or so), it was covered by a thick continental ice sheet. Nick Zentner, geology professor at Central Washington University has a delightful program on the geology of the Seattle area ... www.nickzentner.com. The Washington Natural Resources department has a geology section with an interactive geology map.
It's difficult to tell exactly what type of rock it is without having it in hand. It looks more like sedimentary versus igneous. It may be a granitic rock. With all the glaciation in the area, it may be a well traveled cobble from Canada or from one of the local more cemented sedimentary formations. In any case, what a great find in your garden. Once our health crisis has resolved, you might take it to the geology department at UW for identification. Wishing you and your family good health!
Thank you Carrie for your response.
That was very informative and make a lot of sense.
Looking at some images of river cobble it looks like lava variety.
It was found in the Rainier valley after all.
I’m very fond of this rock and it
will continue to do door duty in my house.
Correction! Like you said... I see it probably is sandstone.
<Grin> Thank you for the update.