I used a scale bar, relying upon the excellent suggestions that Rik kindly provided me.
Incidentally, could anyone please inform me what a more complete description of this pebble would be.
As always, thanks to all.
Stanley

Moderators: rjlittlefield, ChrisR, Chris S., Pau
Good advice about the color. I really thought about that issue, but what you said makes a lot of sense from an artistic point of view.Scale bar looks good. For my personal tastes, the vivid red clashes a bit with the subject. What often works well is to pick a color that already appears in the image. In this case one of the darker reds from the pebble might be good.
All that I know is that I found the pebble somewhere around the Portland, Maine, area (I no longer remember where exactly). I just don't know anything about the red coating. I doubt that I have the pebble around anymore.About the pebble, I can't help much. I'm quite naive about mineralogy. But we have other members who are knowledgeable. I suspect they will want to know a little more about the pebble, especially where it came from and whether the red coating might be artificial or organic rather than a natural mineral.