If you read enough comics, you'll realize that crystal balls were used as a cover gimmick in many genres, over many decades. Let's take a look at a handful of them.
Let's start with Warlord #20 (April, 1979). Of all of the long-running DC series of the 70s and 80s, Warlord may be the one with which I am the least familiar. I have read a bunch, but it never really clicked with me. I should probably give it another chance. I like this Mike Grell cover a lot. He's crammed a lot onto the page, but it doesn't seem too busy. Great design. This was a favourite gimmick for the series as crystal balls were also featured on the covers of issue 16 and 63.
I am sure that somewhere along the way, I have declared myself to be a fan of John Force, Magic Agent. If I haven't, let me do it. now. The cover to Magic Agent #2 (March-April, 1962) is a great example of ACG charm. It's so clean, simple and innocent that it's hard to imagine that they were trying to compete with Marvel and DC. I love Kurt Schaffenberger, and he was a great fit on this type of series. It's too bad he didn't do the interior artwork but Paul Reinman is also underrated.
I cannot discuss crystal ball covers without including Mike Kaluta's awesome cover to House of Secrets #99 (August, 1972). It's obviously an homage to M.C. Escher's Hand With Reflective Sphere, a lithograph first printed in 1935. My guess is that this was on dorm rooms across America back in '72 and that Kaluta's cover resulted in a collective "Whoa - far out, man!". Great stuff and one of the most iconic covers from that series.
Let's visit the superhero genre, as countless heroes have appeared in crystal balls over the years, including the JLA (Justice League of America #21 and #29) and the Legion (LOSH #303). I'll go with Invaders #30 (March, 1979), though, as I actually had it as a kid. I'm not a huge fan of Alan Kupperberg but this cover is decent. I'm not sure that I'd want to be stuck in a confined space with someone called The Whizzer.
For my final selection, I am returning to the horror genre with the cover to The Witching Hour #77 (July, 1977). I am sure that we can all agree that the story title "Coffee, Tea... or Kill!" is awesome, but so is Luis Dominguez' cover. I've seen skeleton Doctors, Santas, Cabbies and Pilots but this is the first Skeleton Stewardess (sorry, Flight Attendant) that I've ever seen. Very patriotic witch, too.
That is just the tip of the iceberg. There are tons of great Crystal Ball Covers out there, so keep your eyes peeled.