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 (HP), but collecting a total of 20 Heart Gems allows for a maximum HP value of 250. Six of the twenty Heart Gems are purchased from Maniankou's Shop while the other fourteen are found in Areas. 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.  Note: The table lists sixteen different Heart Gems. However, two Heart Gems share an internal ID with an existing Heart Gem, listed below. When one Heart Gem is collected, the other one with a matching ID disappears. Only one Heart Gem per ID may be collected. So, there are only fourteen unique Heart Gems to be collected in Areas, so the maximum HP value remains at 250.
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.