How can you get Diamonds in Magikarp Jump?

Since we have another question on How can you get support candies in Magikarp Jump?, it seems like a good idea to have one for Diamonds.

Diamonds are used for unlocking Support Pokémon and Decorations, but seem to be in very short supply - especially considering how freely Coins are given at higher ranks.

What are all the ways to earn Diamonds in Magikarp Jump?

Solution 1:

Here is a list of methods :

• Levelling up your Trainer Rank : 10 Diamonds
• Clearing League matches :
  • 10 Diamonds every 5th battle
  • 25 Diamonds for the League Champion
• Certain Events :
  • "All That Glitters" : 50% chance of 7 Diamonds
  • "Lost Luvdisc" : 4 Diamonds
  • "Name Rater" : 3 Diamonds, requires nicknamed Magikarp
• Sunken Treasure : 21% chance of 5 Diamonds
  • Treasure can be made to appear every 7 hours with Popplio's skill.
• Various Achievements :
  • Achieve achievements : 25 Diamonds at 10, 21, 34, 48 and 64
  • Number of Logins : 25 Diamonds at 3, 7, 12, 30; 50 Diamonds at 70
  • (Unique) Events Triggered : 5 Diamonds at 3, 10, 17, 26, 39
  • (Unique) Patterns Collected : 5 Diamonds at 1, 5, 10, 18, 29
  • Magikarp Fished : 5 Diamonds at 3, 10, 40, 90, 190
  • Social Media Shares : 5 Diamonds at 3, 10, 25, 50, 100
  • Number of Skill Uses : 5 Diamonds at 3, 20, 100, 300, 800
  • Magikarp Forced to Retire : 5 Diamonds at 1, 4, 12, 25, 70
• With the Diamond Miner, 100 Diamonds per day
  • The Diamond Miner costs 250 Tickets, which will set you back about £44.

If you know of any others, please feel free to update this answer.