Two most efficient methods for this would be to make Tar flow downwards in a group of 9 "pillars" altogether, with a one block spacing in between all of them. If Tar happens to pool on the ground that is fine, since the freeze/fire bombs will turn any block of tar/coal (in respective order) into the coal/diamond nodes. This setup will only require one well aimed freeze bomb to the base of the central "pillar" to turn all the other "pillars" into the desired node. Make sure to throw at the base of the pillar on this first throw with the freeze bomb as bombs thrown directly at Tar will pass through them instead. Following that, one well aimed fire bomb to the base of the same central "pillar" will change the coal nodes into diamond.
With only 2 of each type of bomb used to convert two layers of Tar "pillars" (see last screenshot), you can create 72 diamond nodes (assuming each "pillar" is 4 blocks high and all 9 "pillars" are converted) and in total harvest (assuming every node is harvested using a Super Extractor) approximately 72 x 9 = 648 diamond ores or equivalent to 6.5 stacks of 99 diamond.
See: Making Coal.
Naturally Tar will not turn into Diamond directly when set on fire; it will just burn away, as Tar is highly flammable.
As with both of these procedures Diamonds can be made in "infinite" amounts, the balance was changed during game development several times. Since R34 in September 2016 the crafting recipe for Fire Bomb has to be unlocked by crafting Diamond Mining Cells (and Armor-Piercing Bombs like for Freeze Bombs) for example. To craft Fire Bombs, Warmworm Teeth and Shrewdshrew Tails are necessary ingredients too.