Knack 2 has 100 Treasure Chest Locations. Finding all collectibles is required for crystal knacks and gadgets which in turn unlock several trophies.
Every treasure chest contains one RANDOM reward. It can be a part for a new gadget or a colored crystal relic. You can never get the same thing twice. Whenever you open a chest the pool of remaining items decreases. Getting all 100 chests is guaranteed to give you all gadgets and all 4 crystal knacks!
IMPORTANT: Some Chests can only be reached on Normal Difficulty or above! On easy some platforming sections are closed off and you get shortcuts through the levels. Make sure you are playing on Normal / Hard / Very Hard when collecting the chests.
None of the collectibles are missable! You can get them via chapter select (world map) at any time. After opening a chest in chapter select you can quit out to the main menu right away and don't need to reach the next checkpoint. All progress is saved instantly. You don't have to collect the treasures again if you die.
Note: you only need the 7 gadgets and 1 crystal knack for trophies. Meaning you can skip 3 crystal knacks and only need around 50/100 chests to earn the platinum. It's still a good idea to collect them all because the crystal knacks make it easier to complete medals and bonus challenges. Especially Topaz Knack is very powerful which is great for beating hard / very hard difficulty.