"Precious gem rocks have a lower chance to drop uncut diamonds than rare gem rocks, but yield almost twice as much uncut diamonds per minute due to their significantly lower hardness."
I measured the time it took to mine 27 gems at both a Precious and a Rare gem rock, 5 times in a row.
-Mining level 70
-Necronium Pickaxe +4
Using the drop rates on their respective wiki pages, I calculated the average drop rate per minute of uncut diamonds at both gem rocks.
Avg mining time/27 gems at precious gem rocks was 2:49. For rare gem rocks it was 9:40.
The average uncut diamonds drop rate/minute at a precious gem rock is 1.91. The average uncut diamonds drop rate/minute at a rare gem rock is 0.98.
It is currently unknown to me how different mining levels and pickaxes would affect these rates so the edit assumes the mining time for 27 gems of both rocks scales at the same rate.
Rare gem rock time avg. for 27 gems = 9:40 or 580 seconds
Precious gem rock time avg. for 27 gems = 2:49 or 169 seconds
35/100 @ rare = 9.45/27
Diamond rate Rare Gem Rock = 9.45/580s
2/10 @ precious = 5.4/27
Diamond rate Pre. Gem Rock = 5.4/169s
Diamond Drop Rate Per Minute Rare = 0.98
Diamond Drop Rate Per Minute Precious = 1.91
2001:1C02:2C0C:1400:D56B:F741:A5EA:770F 17:23, 12 September 2020 (UTC)