Without examining the stone, that's a good point. I have a light
tent, I'll have to play around with taking photos of it to see if
I can reveal the star that way.
The diffuse lighting that light tents are intended to give you will
not show the star. For that to show up, you need a fairly good point
source of light. That’s why star sapphires often look their best in
sunlight. With your light tent, get the lighting so it’s right for
the ring/metal, etc. They add a strong light from directly in front
of the ring in the form of a focused flashlight or similar pretty
strong, but fairly narrow, beam of light. Often a good penlight held
just outside of the camera’s view will do it. Moving the light source
moves the image of the star, and the brighter that point source, the
stronger the star will show up.
And photoshopping in the star, unless you’ve got considerable skill
with photoshop and perhaps good visual examples to work from, would
be difficult to get to look real. You’re trying to draw a properly
colored and textured image as it appears through a lens, after all.
That would be tricky to get just right.