sorry this is offtopic but I wanted to fully break down eridian rarity for everyone.
(this is all assuming you are opening lance chests frequently and/or killing many many badass corrisive or flame lance)
You can relatively easily find all green eridians in the game (rarest being knoxx released ones - (flame, splat, glob gun, rolling spatter)
As stated above - the green rolling spatter gun is the rarest green eridian, but it’s only roughly as rare as the blue rarity fireball. You can expect to find a fireball about as much as you’d find a decently rare pearl - a Serpens for example.
Of the 3 legendary eridians - The mega cannon is much rarer the fireball but is the least rare of the legendarys (about as rare as finding the rarest pearls in a chest (nemesis, stalker, aries).
The Firebomb and Stampeding Spatter guns are about equal rarity to one another but extremely rare. It is not unreasonable to go thousands of hours farming lance chests and not even see a single one.