Depiction in the seriesThe Crystal Empire - Part 1 as the centerpiece for the Crystal Empire's Crystal Faire celebration. Twilight Sparkle uses her magic to cut a fake Heart out of a crystal block, not realizing until later that the Crystal Heart is a real object.
According to a Crystal Empire librarian, the Crystal Faire lifts the Crystal Ponies' spirits, and the light within them powers the Crystal Heart to protect the empire from evil. During his reign, King Sombra hid the Heart away so it could not be used against him.
In The Crystal Empire - Part 2, Twilight ventures to find the real Crystal Heart, eventually overcoming Sombra's dark magic traps to locate it. Unfortunately, she gets trapped by King Sombra just before claiming the Heart and has Spike deliver the Heart to the Crystal Ponies.With the help of Princess Cadance and Shining Armor, the Heart is delivered to its proper pedestal, and the Crystal Ponies "use the light and love within [them]" to repel Sombra. The magic of the Crystal Ponies and the Crystal Heart transforms the empire, defeats King Sombra, and temporarily gives Twilight and her friends crystalline coats.
The Crystal Heart appears again in the background in Games Ponies Play, Equestria Games, and Twilight's Kingdom - Part 1. In the latter two episodes, it is also depicted in crystal statue in honor of Spike, who the Crystal Ponies venerate for being the one who saved the empire by bringing the Heart to Princess Cadance.
In The Crystalling - Part 1, it is revealed that the Crystal Heart plays a part in the Crystalling ceremony for newborn foals in the Empire. It also prevents the Empire from becoming a frozen wasteland. When Flurry Heart's scream shatters the heart, storm clouds immediately begin to overtake the Empire.
Twilight and Princess Cadance attempt to put the Crystal Heart together in The Crystalling - Part 2 with no success. Sunburst reveals that the Crystalling can repair the heart with the Crystal Ponies' magic combined with the baby's. It works and the storm clouds dissipate.
Depiction in films
The Crystal Heart briefly appears in the background at the beginning of My Little Pony Equestria Girls.
Depiction in comicsThe Crystal Heart appears on the Midtown Comics covers/combined covers of My Little Pony: Friendship is Magic Issue #4 and Issue #5 and on the Larry's Comics covers/combined covers of Issue #17.
In My Little Pony: FIENDship is Magic Issue #1, the Crystal Heart appears in a flashback of King Sombra's childhood. Its magical properties allow it to reflect what's in a pony's heart through visions of the future. It is also stated to be a vital weapon against the opposing power of the umbrum.
In the My Little Pony: Friendship is Magic Issue #34-37 story arc Siege of the Crystal Empire, the Heart serves as a focus in the overarching story. In Issue #34, the Heart is stolen by an alliance of villains formed by Radiant Hope. In Issue #37, following the umbrum's takeover of the Crystal Empire, a restored King Sombra—in defiance of his destiny—uses the Crystal Heart to imprison the umbrum once again.
In the chapter book Twilight Sparkle and the Crystal Heart Spell, recently crowned Princess Twilight undergoes friendship ordeals to discover the "Crystal Heart Spell", and it appears to her on the Heart's surface.
In Princess Cadance and the Spring Hearts Garden, after a very low turnout at the Hearts and Hooves Day Heartsong, Cadance sees that the Crystal Heart is dying. After a vision, she gets an idea about making a garden to bring it back to life.
Mini Pony Collector's Guide page 85 lists Princess Cadance's cutie mark as "Crystal Heart".
In Enterplay's trading cards, the Crystal Heart is featured on series 2 card #49.