Achievement Requirements

Short descriptions of the requirements for all 17 achievements. No level solutions spoiled.

Green Achievements

The green achievements cover the seven chest worlds (Tutorial, Woodland, Sewer, Dungeon, Ruins, Temple, The Void).

Recursion (guaranteed)

Recursion
Unlocked by unlocking the Recursion achievement.
Happens automatically when you enter the first chest that leads to the same room.

Dream Within A Dream (guaranteed)

Dream Within A Dream
Explore a level to a depth of 20 chests.
Just get 20 layers deep. It should happen automatically on late-game levels, but you can easily trigger it on any level with a loop (for instance the level where you unlocked the previous achievment).

I Am Become Chest (guaranteed)

I Am Become Chest
Instantiate 500 rooms.
Should happen automatically after playing long enough. Just enter 500 chests.

Crystal Amaze (guaranteed)

Crystal Amaze
Collect all crystals.
Self explanatory. Just finish all the levels in the chest worlds and collect the purple crystal at the end of each.

Paradox (fairly obvious)

Paradox
Create an impossible situation.
In order to be able to exit a permanent chest (the ones with green energy around them) it has to still be accessible in the same room where you entered it. If you take it out of that room or hide it within a solid object, you get a paradox. You will most likely get it accidentally while trying to figure out the limitations of permanent chests.

Diamond Jubilee (somewhat hidden)

Diamond Jubilee
Collect all diamonds.
Everytime you trigger a paradox (see previous achievement) you get a bonus level with a diamond at the end. Collect all 10 in the chest worlds to unlock this achievement. If whenever you get a permanent chest you engineer a paradox you will never miss a diamond.

Escape (well hidden)

Escape
Find the way out.
On the last level of the chest worlds, The Void - Trilemma, you can not only collect a crystal and a dimond. There is also a third ending which you can unlock by dealing with the vortex. You should first get the diamond and then listen to the vague hints of the rings, which should give you a good idea what to do with the vortex. Major hint: The vortex is a permanent object, complete with green energy around it. Aqua mortua disenchants permanent objects...

Digital Spinning (extremely obscure)

Digital Spinning
Hey, this is fun!
Just mash the right and left arrow keys rapidly in alternation until the character gets dizzy. I have no idea how you're supposed to figure this one out. I had to read through a few pages of forum discussion to find out what I should even do. I guess it's supposed to be some sort of joke/easter egg?

Blue Achievements

Most blue achievements deal with the bonus world The Oobleck Conundrum (left jar at the bottom of the world select screen).

Changeling (guaranteed)

Changeling
Transform 50 ooblecks.
Just transform 50 green hexagons into other objects, which should happen automatically while solving the Oobleck Conundrum levels.

The Oobleck Solution (somewhat hidden)

The Oobleck Solution
Collect all crystals and diamonds in The Oobleck Conundrum.
Finish all the levels and collect the purple crystals. Cause a paradox whenever you get a permanent chest and collect the 5 diamonds in the resulting bonus stages.

Glitch (somewhat hidden)

Glitch
Enter a room that no longer exists.
Duplicate a jar and enter both copies. It does not matter whether you enter and leave the first before jumping into the second, or whether you take the second with you into the first.
I did get it accidentally while experimenting on The Oobleck Conundrum - Transfer: Take the right chest and create a jar, which will automatically duplicate. Jump into one jar and get back out, shattering the jar. Now jump into the duplicate.

Juggling Act (extremely obscure)

Juggling Act
It's good to have a hobby.
Take any three items and put them in one place (the first tutorial level Entryway has 3 boxes at the start). Stand still and hold the up arrow. Now mash the action button to throw the items in the air. After several seconds a custom juggling animation will fire and you will get the achievement. Unfortunately this is another obscure joke achivement that never comes up during normal gameplay and that I had to look up.

Red Achievements

Most red achievements deal with the bonus world The Last Tapestry (right jar at the bottom of the world select screen).

Swapper (guaranteed)

Swapper
Enter 100 cauldrons.
You will get this automatically while jumping into cauldrons. For some odd reason this achievement also seems to count jars, so you'll have a head-start once you get to The Last Tapestry (or you might even have already unlocked it).

Null Context Switch (fairly obvious)

Null Context Switch
Do nothing.
Use a cauldron that leads to itself. Every cauldron leads to a predefined background color. If cauldron A leads to rooms with a red background you just need to take it to one and hop in. On the level The Last Tapestry - Flight you will put two different permanent cauldrons into the same room, at which point any confusion between the two will trigger the achievement.
I did get it a lot earlier while experimenting on The Last Tapestry - Grow by duplicating a cauldron: Take the chest and return with the cauldron. Take the chest again and return with a second cauldron. Now jump into the first cauldron with the second one in hand. When you drop it and hop in, you should get the achievement.

Invalid (somewhat hidden)

Invalid
Arrive nowhere.
Much in the same way you can cause a paradox with a permanent chest, you can cause an invalidity with a permanent cauldron by removing it from its original place after travelling through it.
Example: Chest A with a red background has a cauldron towards blue background. Room B has a blue background and a cauldron towards red. After the first roundtrip, every travel will put you where you last left the other color, i.e. at the cauldron you hopped in. So travelling from blue to red you will always arrive at the cauldron in chest A. If the cauldron in chest A is a permanent one (with green energy around it) and you can access the contents of chest A from the blue background (by having a jarred version or a duplicate of chest A that you can take with you through the cauldron), you can remove the cauldron from the blue side. If you then attempt to travel back to red, your entrypoint (the permanent cauldron in chest A) is missing and you'll end up in a new kind of bonus level with a ruby at the end.

Concurrency (well hidden)

Concurrency
Collect all crystals, diamonds and rubies in The Last Tapestry.
Finish all the levels, cause a paradox with a permanent chest on 5 levels, cause an invalidity with a permanent cauldron on 5 levels. Some of the permanent chests needed for diamonds are hidden inside the invalidity/ruby bonus levels. Some of the stuff needed to get to all the rubies is hidden inside paradox/diamond levels. It starts to get pretty convoluted at this point.

Thread Dancer (extremely obscure)

Thread Dancer
Another ridiculous hidden achievement.
On any level press and hold the action button for 5 seconds until a yoyo animation plays. The description is bang-on as this is yet another obscure joke achievement that you absolutely need to look up. This time you can't even take a guess based on the title.

Happy achievement hunting!

[KonTanu]

Let me add more information.
To achieve this with two items, you will probably need a low ceiling.

[KonTanu]

Juggling Act:
(original) three items
(revised) two or more items

[Gatebase]

Do you have a suggestion? My understanding of "missable" is you can do them while you first play the level or you have to go back. Which is exactly what's the case here. It's just in the context of long-form story-driven games (which this is not) that "going back" often means starting over from scratch. I'd like to convey a priority, a clasification of how likely it is you will need to read the guide for any particular achievement. I'm open to suggestions for a better terminology.

[ekisacik]

I was worried when I first skimmed this looking for one particular thing, because "missable" in other games means "if you don't get this when you can, you have to delete your save file to get it", and I realized that's not true for a lot of these, but rather, it refers to the likelyhood that the player will run into said achievements. I'd suggest changing the terminology to prevent this!