This looks like a small island or rock in a loch to me. The bottom of the diamond would be the reflection of the island in the water.
If it was a misty morning, where the water ends and sky begins would be unclear.
Edit: to illustrate what I mean, I found this image of a small island in a Loch here from a Google image search. The shape is similar to the object.
The view of a little island on Loch Etive from Ardchattan Priory Gardens
HD Calvine image: https://web.archive.org/web/2022081....wordpress.com/2022/08/watermarked-jpeg-2.jpg
General Calvine UFO discussion: https://www.metabunk.org/threads/claim-original-calvine-ufo-photo.12571/
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