Sapphire has a density of 3.98 g/cm³. (Carbon)
Ruby has a density of 3.98 as well (Alumina)
Diamond has a density of 3.5–3.53
g/cm3 (Carbon)
So, Diamond is slightly less dense. (~11%)
EDIT- Boron is 2.46 or 38% less dense than Sapphire/Ruby and 30% less than Diamond. That should equate to less "unsprung" mass, and therefore faster acceleration. But Diamond solid would be stiffer than Boron tube, although you could go almost 40% larger diameter with Boron than Diamond at the same mass... You get into the weeds quickly, but it seems apparent why Shure brothers went to boron tube. And their thin wall Boron tube was larger diameter than their aluminum tube.