I’ve mined every gem in the game with great success, apart from rubies. I’ve given up at this point and just made a transmutation chisel and mined diamonds instead.
It took me forever to find 80 diamonds, but I found 80 rubies in one evening on Alcyon. I dug down to depth 11 under a desert (using atlas and a purchase ruby as a guide) dug long tunnels in 4 directions until I hit lava, then dug up to the max I could reach every third block. Even though there was someone else who had tunneled above me, I found rubies between the two sets of tunnels.
This was all done with lightly speed-forged titanium hammers. My forging skills need work still 
@Honourless how do you define bore boles every 3 blocks? not sure what a bore hole is… and by real digging when does that start and in what form?
right now all i have been trying to do is get as close to an hotspot as i can… dig a 2x1 hole all along for ages in that general area, but i guess this is wrong, i just don’t know what i do when you say dig down to mantle, what you do then when you reach mantle, do you go back up?
Ok say you did your 2x1 tunnel, all along your tunnel you would dig side tunnels branching out, every 3rd block along. At those intersections, you’d also dig straight down to mantel,and then in every side tunnel youd also dig str8 down every 3rd block along. It ensures you reveal every rock possible in the hotspot.
I do the entire 2x1 tunnel first,then the side tunnels (just easier running along it) then finish with the bore holes to mantel after i picked up the gems in the tunnels.
It’s time consuming with iron hammer only, but i prefer to be thorough in the one hotspot rather than jumping to different ones.
If you want to tell if someone has already dug out a hotspot, you first posistion yourself directly in the middle of it and dig down to mantle. The middle of the hotspot is where another player is most likely to have dug. If that’s clear,then you can go back up to 16 and begin your tunneling.
If you want to save time(and hammers) you can use chisel as side tunnels and bore holes instead of diging them out, i prefer to bust rock but that’s just me.
A single 2x1 tunnel is not going to get you many gems at all.
Remember,always use your grapple when digging straight down.
Per the chart at Depths of materials?, you don’t want to dig down to mantle. Instead I’d suggest tunneling at depth 10-11 and digging up for them. Or depth 15 and dig down to 10.
Well i dunno about the chart but i find rubies fairly evenly spready between all depths 16 and below. Including right above the mantle layer. That is to say, not as much as higher but enough to make the dig a few extra blocks worth doing.
My theory is because that depth is rarely touched eventually some rubies are pushed that far down and not touched most of the time.
I farmed 200 today around depth 13.
How?
Just spent 3 hours, found 1 seam got 9 rubies…
Watching this now, being late to the bombmining party makes me twist cringe and scream. I just want my compactor coils 
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Unfortunately that video is out of date, and bomb mining is no longer viable.
The same rules apply though - stay close to the lava line, keep your eye on your atlas, and don’t be put off by other player tunnels. Just because they’ve been there, doesn’t mean they gathered all the rubies. You can even piggyback off their tunnels and dig off the sides of them.
I’d also suggest getting two or three decent hammers, and make sure your skills are setup for mining.
I suggest that anyone interested in making mining experiences more constructive head over to the Awkward mining Co-Op on Maryx. There is a thread in the forums by Merlin that you can find and read. I was stuck using iron tools and had some titanium tools, but not enough to sustain myself completely. I started working with the Awkward miners and within a week I am up to a maxed out mixer 3000power I can make gem tools and compact coal. They have a board that shows what materials they are needed and they trade a miner bundle containing a forged3x3 hammer a durability food and 3 mega strength brews. I honestly believe that anyone can take that and overcome the plateau of iron tools which makes the game way more fun.