Well, I figured it out - once you mentioned materials. The rock object I use is appended from another file ( a 2.83 + file). So I checked the material - it brought in the Principled Shader and the texture, but did not assign the texture to the Principled shader ??
Once I did that, it works fine! It seems so much easier than using a particle system to spread rocks and trees etc, No idea why the .glTF export worked.
It must be tough Jeff, all these Blender changes that may or may not stay around like the Animation Particle Nodes I mentioned above. There are dozens of YouTube tutorials on them and as one person put it : “Particle nodes have been put on hold by the developers, so in the latest versions there’s no longer the option to try these out. The developement is now focusing on Geometry Nodes” All those tutorials of little use now!
That kind of rapidly changing Blender functionality must make it difficult for you.
Stay Safe gryff 