This is my method. For everything but topaz and sapphire I go to mantle depth. I then go 1-2 above mantle and start going nuts with a 3x3 aoe with persist pie and str or spd brew. I then Dig a large box within a mapped hotzone. I then skip 2 rows each tunnel so there is 1 exposed side new on each tunnel lane. With this method I tend to grab a 1-2k gems, maybe more, maybe less per hammer life.
Sometimes I’ll get a hair up my rear and decide to sprinkler and just explode the whole room and leave a hole the size of a fancy gem bomb. If you do things right and get a good run on gems and ore, you can potentially walk away with 1mil in goods if you are able to sale all your rocks/ores/gems/fragments. Those values may be going down a smidge since ores and gems are slowly falling, but you get the idea I hope.
edit: meant to add, so for the price of an extremely high end hammer at 35-50k, you can potentially walk away with 18-19x your investment, if you sale every last item you gathered. Thats looking at top end best case scenario, your returns most of the time will be half of that… closer to 500k on average (i expect this average to go up however since the regen has been getting tweaked and getting higher gem returns is more reliable.)