After looking at several images of Western Black Widow egg sacs online, I agree that it does bear a significant resemblance. If this is correct, it would be of great interest to me. I used to see Black Widows ALL the time when we lived in various urban neighborhoods in So. California, but since we moved to our current location on a large piece of undeveloped land in a canyon in the foothills, I have yet to see even a single specimen (and that's been over 3 years of diligently looking!). Perhaps it is actually from some more common species for my area, but I wouldn't even know where to begin investigating. Anyway, thank you Eric. It at least seems certain to me now that it came from a spider, and I didn't know that much before I posted.

does seem like the most likely thing. The ones I have seen in person were brown and pretty close to spherical, but there is variation within and among Latrodectus species, and they can be whitish and pear-shaped. The size you give is just right.

If you have access to a microscope, you could confirm a spider egg sac by looking for the spiderlings' cast skins--they would molt once before leaving the egg sac... unless that's an exit hole of some kind of parasite, in which case you might find frass/exuviae of that instead.

I know that there is tremendous variation in nature, but I looked at as many images of widow egg sacs online as I could, and for the most part they did seem much more round and less "avian-egg-shaped". Still, in the end, the most likely answer is usually the correct one, especially when one doesn't have any more info to go on. I do not have access to a microscope, but as I said, at least to the naked eye, there did not seem to be any traces of ANYTHING left inside the sac. Thanks for adding your input, Charley.