Ever since My Singing Monsters had first existed, Diamonds are always the main staple of bypassing normal gameplay restrictions, particularly in regards to purchasing special monsters (especially Seasonal Monsters, Rare Monsters, and Epic Monsters), upgrading certain structures, and speeding up breeding/incubation. Players have strived to obtain diamonds in the most way possible in order to advance through the game effectively.
24-Hour Diamond Collection
Continuous collection from each island's Mini-Mines.
This is the oldest method of farming Diamonds, excepting during the time when the old Mine (only on main Plant Island until main Earth Island) was available to purchase. This is a well-respectable method of farming that takes minimal effort besides scheduling time. As long as all applicable islands have all Mini-Mines bought, this strategy is guaranteed to work, provided time scheduling is paramountly prioritized.
Ensure you consistently collect each diamond from each island's Mini-Mine as fast as possible and as consistently as possible. Avoid collecting any other currencies besides Diamonds from Mini-Mines before continuing any other gameplay, so as to avoid inconsistent earning of diamonds. Also memorize the pattern of collecting each island's diamonds.
Assuming full island unlocks, the recommended way is to start 24-Hour Diamond Collectioning on the new Carosel-style Map is by starting with Cold Island, then Mirror Cold Island, then Mirror Air Island, then Air Island, then Water Island, then Mirror Water Island, then Earth Island, and then every other island in order. Either that, or start with Ethereal Island, then going backwards in order towards standard Earth Island, then Mirror Earth Island, then Mirror Water Island, then Water Island, and so on repeating the negative-coswave-like pattern.
Avoid starting the 24-Hour Diamond collection at near night time, if possible. Otherwise this can literally worsen real-life sleeping patterns.
Since August 2017, all Wublins are available in unlimited amounts, assuming physical space is still available. Thus, Zynth Farming is possible as of that time. There are no caps to how many diamonds are produced per Wublin, just a 7% chance to generate diamonds per Wublin regardless of type. As Zynth has the smallest footprint (2x2), alongside Thwok, Zynth is by far the best choice to make masses of diamonds. Just be sure to collect every 6-12 hours to refresh more Zynth collections.