There are 125 blue, pink and gold crystals making a total of 375 to collect. They are all relatively easy, but collecting them can be made even simpler by equipping the Energy Wings which pull crystals towards you. If you’re struggling to find the last few crystals in an area you can land on a platform and check your area map to see where they are in relation to your position. Some may seem difficult to find, but usually it’s a matter of flying into a cave or tunnel to get to them.
Note that collecting all blue crystals unlocks Sporelle where there are more pink and gold, and collecting all pink crystals both unlocks the fan to Ascension where you can get the remaining gold crystals and releases the previously unattainable one in Arctasia.
"You'll also need the Fiery key to open the gate in MagmaCore to get all gold crystals on that map"
Thanks and credit to janiwesterling for pointing this out.
Breakdown of crystal locations:
Arctasia - 45 blue, 5 pink, 1 gold.
Spiral Heights – 20 blue, 20 pink, 20 gold.
Oasis – 50 blue, 25 pink, 14 gold.
Sporelle – 30 pink, 20 gold.
MagmaCore – 10 blue, 45 pink, 40 gold
Ascension – 30 gold.