Bruh 500+ for the win. You know about crystal rarity and mythstone rarity right?
They go up in tiers per 100 floors. For floor 100, calcite will never drop, floor 200, angelite will never drop, floor 300, quartz never drop, floor 400, diamond never drop, floor 500, larimar never drop. Same story for mythstones but from hero to gift.
The common crystals drop instead of being calcite to larimar , it will turn inro larimar to peridot as common drop in floor 500. For mythstone, instead of hero to gift being common drop, it be wisdom to death i think?
Also in floor 500, what would have been jasper in floor below 100 turns into obsidian drop at floor 500. I gotten quite a few obsidian up there. Also amethyst, lot of ruby,zircon,garnet,jaspers,ambers,topazes, etc etc. Also for mythstone side, i got quite a few elixir, zenith, master, time, return, fortune, quest, etc etc.
Basically best prority for farming crystals/mythstones is journey your way to floor 500+ with ease (must be easy to complete) for the best chances of rarer crystals. Legend maps/eternal maps affect this greatly too. I believe every 100% on crystal/mythstone rarity means a tier rarity up? Correct me if i am wrong but i assume so? Not a tier up by 100% but by some % maybe as well.
Best way to get the most crystal/mythstone is crystalline, mythical and item drops 300% (200% max +100% hidden item drops for having hirling) as well as being up in the easy floor 500+ (provided you find it easy). Generally for enhancing farming experience , max gold find, max luck, max item drops, have fortunate, treasured, accomplished , enshrined, dealer and hunter (hunter ring if you dont have his acension perk) acensions.
Ofc, you have to farm the acensions too with exp boosting gear but thats a different thing for farming and meant for a short term farming gear. Hope this helps 
Another tip, after floor 500, there won’t be anymore crystal/mythstone rarity boost and that is the maximum you can get for crystal rarity boost (plus crystalline (5) or (6)). To add crystal rarity after that permanent boost from crystalline with floor 500 rarity, legend maps with crystal/mythstone rarity or eternal maps. I think the maximum you can get is a perfect eternal map with all affixes rolled to max with crystalline (6) and be at floor 500 but don’t go extreme. Recommend is just 1 crystalline /mythical with floor 500 rarity and if you find occasional legend map, go ahead when crystal/mythstone rarity is near perfect roll. Also to further enhance this whether your extreme or not, when you have occasional mythstone/crystal boost from ads, you can use it with floor 500+ rarity and if you have legend map, go ahead. If you have eternal map, also go ahead.