In order to unlock this feature, you have to unlock the City Reputation system first at Adventure Rank 25, then reach level 2. This lets you talk to the blacksmith in that city to "mark" Rich Ore Reserve locations, that also comes with some Crystal Chunks on the side.
There are several NPCs that can tell you the locations of rich ore reserves, but the easiest one is the blacksmith. Available in every city, you can talk to the blacksmith and select the "Any tips on finding ore in Mondstadt?"
Talking to NPCs will only tell you the locations, not spawn new ones. If you are unsure if the magical crystals have spawned or not, then you can ask for their location by talking to the npc in the area that you want to look for.
They will then give you several mining spots with rich ore reserves that has Magical Crystal Chunks between 3~5 spots.
When you get there, you will see plenty of crystals there. Magical Crystal Chunks seems to spawn in more or less the same locations, and they respawn each day, 24 hours after you mined them (unsure). There's a total of 16 (6 in Mondstadt, and 10 in Liyue).
Magical Crystal Chunks
Magical Crystal Chunks are used to craft Mystic Enhancement Ores, a weapon exp material. It's the same as the version using Crystal Chunks to make, except you are limited to only crafting 30 Mystic Enhancement Ores per day if you use Crystal Chunks.
Using Magical, it requires 10 resins and 3 Magical ores to make 6. But it only takes 5 seconds each, and you are not limited to how many you can craft per day.
When you reach level 2 of your Mondstadt's City Reputation, you can talk to the blacksmith near the city gates to get the option for rich ore deposits.
In Springvale, you can talk to Draff for some more locations. At night, he shows up in front of the tavern, drunk, so you won't be able to get any answers from him. You'll need to change the in-game time by selecting the "time" feature below the mail icon from your menu. This might be the case with other npc's as well, where you can only talk to them during certain times of the day.
You can talk to him in the morning/afternoon to mark the available ones in the area.
Doolan is in the Thousand Winds Temple. He might only be in this location because of the event quest moving him there. If he is not in this location, it's slightly further to the west of the Thousand Winds Temple.
Davy the bard is at Brightcrown Canyon, near the cliff. Talk to him to mark the available spots in the area, if there are any.
You can find the Liyue blacksmith marked in the map in Liyue Harbor, he's in the east part of the city. Similar to the one in Mondstadt, you might need to reach level 2 of the city's reputation to unlock this option.
You are able to talk to Pan Guan'er, in Jueyun Karst, north west of Liyue Harbor, for some rich ore deposits as well.
Shizhuang the strong
Lingju Pass is west of Liyue Harbor. This area has some spots with plenty of mineable rocks, and interactable baskets for more ores. The npc Shizhuang the Strong might also have some more to share.