With our latest two wearables I’ve found a problem with the rendering of objects in the Marketplace.
Take a look at this collage - two different wearables in Blender - rendering is fine in Eevee mode in both cases. Even ingame, in the Editor, in Babylon.js or the Windows 3D viewer they look ok. But in the marketplace they look completely different.
The “DripWatch” has a complete black surface in the marketplace and I could only find one difference to the “Mana Necklace” - the Metallic slider in Blender. I think the shader used in the marketplace doesn’t render the wearable correct when the slider is set to 1.0 (full reflection) or has a general problem with reflectivity at all or just doesn’t have an enviromental reflection map implemented yet, like Eevee has. Since I recentely programmed such a shader from scratch myself, I am somewhat familiar with this problem, although I can only guess what the cause might be.
Perhaps you can pass this to the Devs to take a look at. And I would recommend all creators not to set the metallic slider too high until this is fixed, better leave it to 0.5 - so the Roughness - unless you must change the reflection of your object (which, in this case made sense as gold/diamonds is a very reflective surface as you know 
)