Crystal Heart
Crystal Hearts are high-importance collectibles in Celeste. Their primary purpose is to unlock different chapters locked by Crystal Heart Gates; they are also a prerequisite to unlocking all C-Sides and Variant Mode. They also serve as chapter completion objectives for all B-Sides, all C-Sides, and Core A-Side. Blue Crystal Hearts appear in every A-Side; Red Crystal Hearts in every B-Side; Gold Crystal Hearts in every C-Side, and a Gray Crystal Heart in Farewell. Each Crystal Heart, minus C-Side ones, also has a name specific to itself, and an associated achievement with the same name.
After a heart is collected, Madeline slows down, and the game continues to run for 135 frames (~2.25 seconds) before she completely stops. It is possible to die during these 135 frames, for instance if Madeline dashes into spikes during this time. After the collection cutscene depicting the heart in the middle of the screen, the heart is saved, and will remain in the save file permanently.
A-Side Crystal Hearts
Crystal Hearts in various A-Sides serve as well-guarded secrets, often hiding at quite a significant distance away from the main path, or requiring otherwise tedious tasks to be performed. These Crystal Hearts are blue in color; each one has a unique name and achievement associated to it.
An exception to this rule is Core A-Side; the heart acts the same way as a B-Side or C-Side heart, being at the end of the level, and collecting it ends the level. Such hearts also exist within various Celeste custom maps.
B-Side and C-Side Crystal Hearts
Crystal Hearts appear at the end of a Cassette Block room in every B-Side in Celeste. These Crystal Hearts are red in color, and also have a unique name and achievement associated to it.
Every C-Side also contains a Crystal Heart at its end. Unlike the B-Side crystal hearts, these often aren't located at safe locations, and Madeline can die whilst collecting them. These Crystal Hearts are gold in color, and unlike A and B-Side Crystal hearts, do not have an associated name or achievement.