The idea is that you would cross your fingers and hope that one dropped and if (when) it didn’t you’d at least hope you got some higher higher crystals (lets say Jaspers and above) that you could then upgrade to Obsidian.
Same concept as High Runes in Diablo 2 – you would probably never see a Zod rune drop, but if you played a lot and saved all your other high runes you might be able to convert up to one eventually.
Frankly with Crystal legendary items being possible, for many slots you are probably better off getting a Crystal Legendary for that slot and then customizing it to fit what you want rather than working up to an Obsidian. The exceptions would be when a specific legendary already has a fair number of Legendary/Set affixes you want such that trying to customize another (crystal) legendary or epic is going to cost as much or more in terms of applying Legend/Set affixes.
You can find legend items with a crystal affix on them (called “deviant item”) if you’re lucky, saw some players ingame who already got some of them
You’re not supposed to reach obsidian by fusing from calcite, ofc it would be too much, but calcite ain’t the most looted crystal, and you can find easily some #10-14 crystals
Crystal affix is supposed to be an endgame feature (let’s being honest, w’re playing an hack n slash, like diablo, how the hell would you be godlike in a week and would it be good if we could be full crystals in 1 month? nope, it’s about playing hours and hours, that’s how does work a hack n slash)
So yeah you’ll prolly spend 121217 hours to get a crystal, but that’s worth, and no one force you to reach crystals items, you can still be powerful with epic/legend items