Are you thinking of trying some new bases?
Yes, your observations inspired me.. I didn't imagine that there could be polished stones with a surface better than glass. Glass is commonly used for precision tops, I too use it for my tops with an external ball tip. But it would be nice to have a stone spinning surface. I should make some tests and see if diabase could be good for me too: my ball tips are made of ruby, sapphire or similar very hard materials.
I recall the very impressive test matrix you did with a range of tip and base materials. Looking forward to your tests with stone bases, as I also like the way they look and feel.
Just looked through some engineering literature on bearing materials. Looks like relative hardness across a bearing is only one consideration among many. The relationship between friction and wear resistance is also complicated.
So depending on what you're after — spin time or wear resistance — diabase might or might not be better than glass for your tops. The commercial name for my diabase was Absolute Black.
To put my experience with diabase in perspective, my tips are all proprietary ABS plastic with a low Mohs hardness of ~2. The minerals in diabase are all MUCH harder. Contact radius of curvature varies from 1.6 to 5.5 mm, with the 1.6 mm tip generally giving the least travel and longest spins.
Since most of my tops weigh well under 100 g, the bearing loads are generally quite small. Wear rates are surprisingly low but definitely increase with load.
At Mohs 9, your ruby, sapphire, and tungsten carbide tips are all much harder than any of the major minerals in diabase, most if not all of which are somewhat harder than glass.
Unfortunately, hardness is a very complicated physical property. Most common hardness scales are based on scratchability or indentability, which aren't quite the same thing. We're also interested in what will
drill what at typical top weights and speeds. That's surely even more complicated than hardness.
