Normally, an amethyst geode needs a lot of blocks to spawn in bc it's an underground structure. But it can generate in only a block if spawn by "/place".

STR:
Step 1: Load in a world, superflat is recommended.
Step 2: Place the first "Always Active" repeat command block that said: "/place feature minecraft:amethyst_geode ~ ~10 ~".
Step 3: Place the second "Always Active" repeat command block that said:

"/setblock <X-pos-of-1st-repeat-command-block + 5> <Y-pos-of-1st-repeat-command-block + 15> <Z-pos-of-1st-repeat-command-block + 5> <block>" (For example if the 1st repeat command block (the repeat command block that said "/place feature minecraft:amethyst_geode ~ ~10 ~") is in 0 -60 0 then since 5 is 5 more than 0 and -45 is 15 more that -60, the 2nd repeat command block will said "/setblock 5 -45 5 brain_coral_block")

, then wait. -> You will see that an amethyst geode will spawn