Hi Wolf, your photos are certainly not inadequate, they have definitely improved since you first joined the page, and I appreciate you asking.
I think the first thing I would mention is the area around the marble. When you are photographing a marble, you want the marble to be the focus, so any crazy background color or texture will take away from the focus of the photo.
In this screenshot below I would remove (crop out) the area outside the red rectangle, especially if it's a screenshot and there is black area above and below your screenshot. All that extra space around the marble makes for a lot of scrolling for just a few photos here.
I hope that helps.