After wandering around in the Maple world, I found the following pattern for herbs and veins locations.
First of all, what determines which herbs and veins are in a map are the level(s) of the monster(s) in those map, split in 3 level ranges. If there are monsters of different ranges, the highest range is used.
Level 1-29: No herbs and veins are spawned.
Level 30-99: Which herbs and veins are spawned depends on the last number of the monster's level. For example: 30 → 0, 35 → 5, 39 → 9, 89 → 9 ect.
Then use the following table: Number Herbs and Veins color
0,1 Silver & Magenta
2,3 Blue & Brown
4,5 Emerald & Gold
6,7 Aquamarine & Red
8,9 Black & Purple
• If there are multiple monsters in the map, herbs and veins for all those monsters are spawned. If the levels of the monsters results in herbs and veins of non-adjacent color, all colors between are also spawned.
For example, in a map spawning level 81 and 84 monsters, the herbs and veins that are spawned are: Silver & Magenta (from lvl 81 monsters), Emerald & Gold (from lvl 84 monsters) and Blue & Brown because it's in between.
Level 100-200: All these maps spawn Emerald, Gold, Aquamarine, Red, Black and Purple herbs and veins, regardless of level.
Sometimes there are less herb and vein spots than allowed herbs and veins, so it is possible you won't find a Purple Vein in the first level 100+ map you enter, but you'll encounter them in other channels.
Heartstones and Golden Flowers are spawned randomly in any map where herbs and veins spawn, albeit with a very low rate.
In this table you can see which ores/seeds/flowers drops from which herbs/veins:
Color - Ores - Flowers & Seeds
Silver - Silver & Opal - Marjoram
Magenta - Orihalcon & Amethyst - Lavender
Blue - Steel & Sapphire - Rosemary
Brown - Bronze & Adamantium - Mandarin
Emerald - Mithril, Emerald & DEX Crystal - Lemon Balm & Peppermint
Gold - Gold & Topaz - Jasmine
Aquamarine - Aquamarine & Diamond - Tea Tree
Red - Garnet & Power Crystal - Chamomile
Black - Black Crystal & Dark Crystal - Patchouli
Purple - Litium, LUK Crystal & Wisdom Crystal - Juniper & Hissop