Gems appear to be in the same locations between difficulties, but treasures differ.

Level 1-1:
Green: second enemy you find
Green: enemy under blocks
Blue: hidden enemy in arena
Green: block in arena

Level 1-2:
Green: Third enemy you find
Green: Fourth enemy you find
Blue: enemy behind block
Green: enemy above blue gem enemy

Level 1-3:
Green: first enemy you find
Blue: enemy just below spikes
Green: hidden enemy in arena
Green: hidden enemy after arena

Level 1-4:
Green: first shield enemy
Blue: at top of waterfall with stone on either side
Green: from hidden enemy
Green: in block at end of level

Level 1-5:
Green: third enemy
Green: first shield enemy
Blue: falls from ceiling like a rock after first shield enemy
Green: in blocks at end of level

Level 1-6:
Green: first enemy
Green: enemy hidden behind blocks after arena
Blue: Shield enemy under block
Green: Toad at end

Level 1-7:
Green: block after first two enemies
Green: first shield enemy
Green: spider in Z-shaped corridor
Blue: up spider string at end of level

Level 2-1:
Blue: first enemy (at start)
Green: third throwing enemy
Green: fourth throwing enemy
Green: in ceiling after fourth throwing enemy

Level 2-2:
Green: second enemy
Green: third enemy
Green: fourth enemy (in tree)
Blue: last enemy (in tree)

Level 2-3:
Green: first enemy
Blue: shield enemy in one-tile-tall passage
Green: throwing enemy above shield enemy.
Green: shield enemy after arena

Level 2-4:
Green: first enemy
Green: third enemy
Blue: hidden area left of arena entrance
Green: second enemy after arena

Level 2-5:
Green: second enemy
Green: vine enemy after hole in the ground
Green: block behind red vine beetle
Blue: one-block wide hole in ground

Level 2-6:
Blue: hidden enemy at start
Green: second throwing enemy
Green: second worm enemy after arena
Green: hidden area above second worm enemy after boss

Level 2-7:
Green: block near start
Blue: big leaf enemy
Green: leaf enemy after big leaf enemy
Green: toad enemy near end

Level 2-8
Green: first enemy
Blue: very top-right of the starting screen
Green: first enemy after arena
Green: enemy after second arena

Level 3-1:
Green: box near start, next to spikes
Blue: keep hitting the boot the fisherman is pulling up
Green: fish underneath boxes and between spikes
Green: hermit crab just before exit

Level 3-2:
Green: second enemy
Green: third enemy
Blue: hit fish in arena
Green: last enemy

Level 3-3:
Green: second lurking enemy
Green: lowest enemy in arena
Green: block after arena
Blue: last enemy before exit

Level 3-4:
Blue: block hidden in sand at start
Green: block behind seaweed, near first 2 spiked enemies
Green: last of the spiked enemies in the narrow passage
Green: last enemy before exit

Level 3-5:
Green: first enemy
Green: second group of jellyfish
Green: first blaster enemy guarding passage
Blue: gap in ground in arena

Level 3-6:
Green: first hermit crab enemy
Green: lurking enemy below first hermit crab enemy
Green: last blaster enemy before the shipwreck
Blue: hermit crab enemy hidden behind shipwreck

Level 4-1:
Green: second enemy
Green: second spear enemy
Blue: in ceiling after arena, look for the one pillar that's dark
Green: second enemy after arena

Level 4-2:
Green: last vine enemy in group
Green: shield enemy in front of vine enemy
Blue: waterfall that's only 1 tile tall
Green: last enemy

Level 4-3:
Green: first enemy
Blue: block at bottom of level
Green: column of blocks
Green: spear enemy after arena

Level 4-4:
Green: first enemy
Blue: L-shaped passage at bottom of level
Green: two blocks at top of level
Green: last shield enemy in level

Level 4-5:
Green: first enemy
Blue: catch the sword of the red arena enemy after killing them, then break through the spikes to the left.
Green: swordsman enemy after arena
Green: shield thrower near end of level

Level 4-6:
Green: second enemy
Blue: jumping spear enemy (kill it before it jumps off into the pit)
Green: first crate after arena
Green: last enemy before exit

Level 4-7:
Green: second enemy
Blue: below the second twinned enemy
Green: hidden behind pillar
Green: twinned enemy after arena

Level 4-8:
Green: first flying shield enemy
Blue: below ground in waterfall
Green: second flying shield enemy
Green: third flying shield enemy

Level 5-1:
Green: first enemy
Green: second lava lizard
Green: second lava lizard after arena
Blue: shelled enemy next to brick. Break the brick first so it drops two spaces, not just one.

Level 5-2:
Green: third enemy
Green: shelled enemy
Blue: shelled enemy near blocks (hit a block that the enemy is on top of)
Green: last enemy before exit

Level 5-3:
Green: in blocks after arena
Green: enemy after arena
Green: enemy after second arena
Blue: in block near exit

Level 5-4:
Green: block at start
Green: block in ceiling of first arena (where there's no background stone)
Green: gap in ceiling after second arena
Blue: bottom middle of third arena (under blocks, has to be broken by boss)

Level 5-5:
Green: first enemy (go fast)
Green: first blocks
Green: first shelled enemy (knock the bomb at it)
Blue: second shelled enemy (knock the bomb a screen earlier at it)

Level 5-6:
Green: first enemy
Green: second twin enemy
Blue: twin enemy after arena (keep it from falling off-screen)
Green: enemy between blocks

Level 5-7:
Green: second shelled enemy
Green: first horned enemy
Green: lava lizard near horned enemy
Blue: kill arena enemy by dropping blocks on it (key is in blocks)

Level 5-8:
Green: block near start
Green: second enemy
Green: shelled enemy
Blue: block near exit

Level 6-1:
Green: third chain enemy
Green: second chain enemy after arena
Green: jumping spear enemy
Blue: ceiling near exit (the big block to the left is shorter than the others.)

Level 6-2:
Green: lower enemy near start
Green: enemy under blocks
Green: first enemy after arena
Blue: third banner after arena

Level 6-3:
Green: blocks near start
Green: enemy on ledge
Green: enemy in passage
Blue: ceiling near exit (covered by pipe)

Level 6-4:
Green: second shield enemy
Green: enemy in arena
Green: block after arena
Blue: the pumpkin

Level 6-5:
Green: lower enemy at start
Green: shield enemy after arena
Green: mummy enemy near bricks
Blue: mimic brick near exit

Level 6-6:
Green: second enemy
Green: spike enemy in passage after arena
Blue: behind pipe immediately after second arena
Green: twinned enemy after second arena

Level 6-7:
The cauldron enemy is a bit bugged at high framerates. What worked for me is hitting shift-tab a few times to bring up the Steam overlay. (It might take a few tries.)
Green: second enemy
Green: hand in Z-shaped passage
Blue: in floor in z-shaped passage (under the bones)
Green: enemy near exit

Level 6-8:
Green: second enemy
Blue: pit after arena
Green: ghost after arena
Green: enemy near exit

Level 6-9:
Green: second flying enemy in open area
Green: chain enemy just before spikes
Blue: shelled enemy (kill using fireball from flying enemy)
Green: flying enemy in passage

Level 6-10:
Green: laser skull after arena
Blue: last mummy in group after arena (hit from above to keep from knocking it off)
Green: ice wizard after laser skull
Green: fire wizard after laser skull

These spoilers are divided in two in case you just want a pointer; the first line shows the level to look in, the second the actual location in the level.

Normal difficulty:
World 1:
1-5
Second rockfall from the end, in the ceiling

World 2:
2-8
In pit just before exit

World 3:
3-5
behind blaster enemy behind block

World 4:
4-2
ceiling at exit; look for the space lacking vines

World 5:
5-6
lava before exit that's not bubbling

World 6:
6-4
the pit where the pumpkin emerges (you can just head back after killing the pumpkin)

Hard difficulty:
World 1:
1-7
Behind the blue gem

World 2:
2-7
Under big leaf enemy

World 3:
3-1
Under fisherman

World 4:
4-3
Gap in floor under jumping spear enemy (it will jump over the gap)

World 5:
5-8
Above second rockfall

World 6:
6-6
Behind pipe near exit