Heart Gems, known as Konpeitou, コンペイトウ in Japan are special treasures Starfy and Starly collect in order to gain more health. In The Legendary Starfy, they appear in large blue treasure chests and appear as diamond-shaped gems engraved with a yellow heart-shape.
In Densetsu no Starfy 4, obtaining a Heart Gem adds ten health points on to Starfy and Starly's health. Unlike The Legendary Starfy, Heart Gems do not appear in blue treasure chests in Densetsu no Starfy 4. Some appear through Tobira Majin, while others can be found prior to completion of the main game. Starfy and Starly begin with 50 health points, but collecting a total of 20 Heart Gems allows for a maximum HP value of 250 health points. Six of the twenty Heart Gems are purchased from Maniankou's Shop. One Heart Gem becomes unavailable at some point so that the maximum amount of HP doesn't extend to 260. When Starfy or Starly finds a Heart Gem in an Area, a short tune plays.
Below is a table of Heart Gems locations for Heart Gems found in stages. 
In The Legendary Starfy, there are fifteen big blue treasure chests containing 15 Heart Gems. These are found in 15 secret Areas. If Starfy collect 3 of them, an extra heart is added on to the health bar. When all Heart Gems are collected, Starfy and Starly gain a total of 10 hearts.
Heart Gems are also stored on Bunston's Collection. There are three spots for Heart Gems. When a third Heart Gem is collected the Heart Gem icons are removed. When Starfy obtains a Heart Gem, a short tune called You Got a Heart Gem! plays that is different to the tune for obtaining a Heart Gem from Densetsu no Starfy 4.
Below are a list of areas which contain Heart Gems. 
- Stage 1-7 (Gluglug Lagoon - Area Secret 3)
- Stage 2-5 (Hotcha Springs - Area Secret 1)
- Stage 2-6 (Hotcha Springs - Area Secret 2)
- Stage 3-6 (Chillydip Cove - Area Secret 2)
- Stage 3-7 (Chillydip Cove - Area Secret 3)
- Stage 4-5 (Sogwood Forest - Area Secret 1)
- Stage 4-6 (Sogwood Forest - Area Secret 2)
- Stage 5-5 (Glitzem Grotto - Area Secret 1)
- Stage 5-6 (Glitzem Grotto - Area Secret 2)
- Stage 5-7 (Glitzem Grotto - Area Secret 3)
- Stage 6-5 (Skydye Heights - Area Secret 1)
- Stage 6-6 (Skydye Heights - Area Secret 2)
- Stage 7-5 (S.S. Logwater - Area Secret 1)
- Stage 7-6 (S.S. Logwater - Area Secret 2)
- Stage 8-6 (Planet Bunnera - Area Secret 2)
- Exact locations of the Heart Gems found in stages in Densetsu no Starfy 4 with map locations marked on the screenshots: prior to completion and post-game.