If you go to Kid's or a helmet's page, the diamond powers will be described in further detail. For example, on Kid's page: 

https://kidchameleon.wikia.com/wiki/Kid

The diamond powers are listed as:

On cyclone's page:

Although the descriptions do not go into as much detail as they could, I hope this is helpful.

As I've seen it, that "Circle Of Doom" isn't usually too good a power. The Kid, Iron Knight, and Maniaxe all have it, but it can only last up to 10 seconds, and has a limited capacity concerning enemy hit points, so it might get used up faster than that. I'm likely, though, to want Maniaxe using it in part of "The Final Marathon." Those Mini-Hopping Skulls are so hard to hit before getting hit by them. But that means I have to be careful to refrain from collecting more than 49 diamonds, and "The Final Marathon" actually has quite a few diamonds.

I feel it hitting me now to talk about Micromax. His diamond powers two, are both called "swift mini-snakes." They spiral out from you sort of like the Diamond Snake of a few others, but move a little faster and spiral a little more neatly. The 20-diamond version for "Little Mike"(nickname I like to call Micromax) is functionally a lot like "Circle Of Doom." It doesn't last long, and if there's an enemy with more than 1 hit point you might see less diamonds in it, showing its limits in harming enemies. The 50-diamond version is better, but it sure can be a waste if the enemies near you are few and easily handled. I'm most apt to use them in "Alien Isle," 4th quarter of the game. A teleporter at the bottom of the deep left section can be used to refresh Prize Blocks and regain diamonds helping Maniaxe work up quite a few extra lives, but I prefer to get 90 or a few more diamonds and save them for use as "Little Mike." He may need to try first a 50-diamond version and then two successive 20-diamond versions to get rid of those lousy U.F.O.'s, but he can grab the sides of the walls in the near-exit section, so if I can just keep from accidentally hitting a Maniaxe helmet, I can get to the exit without using those lousy elevators.

Berserker may have overall the best assortment of diamond powers. For 20 diamonds it's a temporary personal shield--I like to use that in "Windy City," right at the start of the level, because that permits running right through the nearest Tank, avoiding being hurt by its gun-blast, and makes finishing the level with a no-hit bonus possible. The 50 diamond power, Wall Of Death, is pretty good considering. Those waves of diamonds sweep back and forth for as long as about 22 seconds, and no enemy is safe. A level as infested as "The Hills Of Forever" is where I'm most likely to use that one.