1 Rainbow Gem will be received upon first clear of each color rush.
This event occurs two times on Monday (and every Friday since 1st of April 2016, too) and lasts 30 minutes each. Guerrilla Turtles Times change every Monday and Friday, but on a given day are always 10 hours apart. See Guerrilla Turtles Times for info on how to calculate the time the turtles will arrive (or use the following gadget: ).
The event above is an example of a black turtle rush which occurs each Monday. Analogous events occur for other four colors on Friday.
The first stage contains 5 Treasure turtles, the middle stages, one Daimyo and 2-3 Treasure ones, and the last one, one Elder and two Daimyo. Two special rare stages can appear: one which duplicates the final one, and the best one - containing just the Elder turtle, virtually guaranteeing its drop.