It does seem that seams take 110% damage from bombs 
rocks also only take 75% edit: yeah… looks like this is accurate
I don’t believe in secrecy, so here goes:
Rock blocks (Hard Blocks) have 1200 health to begin with.
Seam blocks have 1800 health to begin with.
This is then multiplied by 3 because of the gem worlds being t4.
So Rocks have 3600
Seams have 5400
Additionally, they have 400 armour, so subtract 400 damage from any hit on any block on a t5.
That results in needing to do 3600 damage / 5400 damage respectively, and you can work out your damage (using a hammer at this stage) by doing (damage-400)*number of hits it takes
However, using bombs introduces more complexity to this, as seam blocks seemingly take 110% damage from aoe attacks, and hard blocks (rocks in this case) take 75%
So.… a bomb that did 1000 damage according to the tooltip would do (1000-400)*.75(450) damage to rocks per hit, and (1000-400)*1.1(660)
With all this complexity, we want a bomb that will leave all the seam blocks, having broken all the rock blocks.
I’m going to begin the checking with silver bombs, as silver and gold are relatively abundant.
SILVER BOMBS – They do 1200 damage according to the tooltip. This means they do 600 damage to rocks and 880 damage to seams
So, theoretically silver bombs, if you have no skills, work great with 6 bombs breaking rocks and leaving seams with 120 health.
Does any of this seem correct?
If someone could test this, emphasis on no skills into damage, that would be awesome.
Obviously you can do your own calculations based off this info, if it ends up being correct.
EDIT: Looks like im off on the damage somewhere… its much less that I thought
Edit edit:
I wasn’t taking into account the impulse armour and reduction of the blocks. Hard blocks (rocks in this case) have 400 impulse armour, so that takes another 400 damage off, and only take 50% impulse damage.
The new formula would be (damage-800).5.75 for hard blocks and (damage-800).51.1 for seam blocks
So, with a silver bomb and no extra damage its:
(1200-800).5.75 = 150 damage
(1200-800).51.1 = 220 damage.
Hopefully this is right now.
Then to get the damage with your overall damage multiplier, do damage + (damage * multiplier%)