Eight foot diameter metallic disk with pointed crown-like prominences along its rim, cruising SW to NE 100 yards above head.

I was walking along a gravel trail from west to east at the Wildhorse Agricultural Buffer on the north edge of Davis, CA. It's a re-wilded nature area along a golf course. The sun was still high. I stopped to video bees swarming around a large 15' foot tall bush. Turned off the camera and looked up. There was a metallic object silently moving from SW to NE maybe 100 yards above me. At first I thought it was a balloon, but no; it was too big, probably 6'-8' in diameter. It reminded me of the type of clam shell with spikes or protuberances coming out of the rim. Also looking like the animal, giant sea star, but the arms (spikes) were very short. It looked organic and crown-like, but was dark metallic. It looked like copper or bronze with a dull matte finish. It was cruising along to the NE at only 200-300 feet above the ground, wavering, but never flopping over and maintaining it's altitude. Here's the weird part: I raised up my phone, turned on video and said "Thank you" out loud, because I knew this was really unusual. Held up the phone to the object, observing both it and my screen at the same time, but the object would not show up on my screen. Even though I was staring at it in the sky. I zoomed in and out, noted where it was with reference points of cloud edges and nearby trees, but it just wouldn't appear on my phone screen. I kept shooting and watched it cruise out of sight to the NE at maybe 30 to 50 mph. I walked back to my car and checked the footage. It wasn't there. When this first caught my eye, I felt elated. Now I felt uneasy. It was like this thing saw me and didn't want me to take its picture. I felt like a lab rat, and I've never felt that way before. I took the images home home and ran them through photoshop, trying out filters that can find edges of things, but they found nothing. It's my word against the world on this one, but I know what I saw, and I wasn't hallucinating. I will keep looking skyward. I'm a private pilot. That doesn't make me an aviation expert, but I've got a somewhat trained eye for estimating distance, size of craft, elevation, and speed. There are weird things flying around up there in the sky.