Cash. It's simple. Each coin or gem you make contact with will add 5 cash to your cash gui. Your cash gui will glitch for certain when you enter the server and not display any cash at all, along with your inventory, but this can be easily fixed by resetting. The color of the gem you collect does not affect the quantity of cash you are getting in any way.
And, by earning alot of cash say about 35 cash you could buy obsidian chips, (but you need to be under level 100) to level you up once. But you need 2 of them in order to level up.
Notable places to find cash include the temple in the forest and the cave above the desert (Where you get obsidian chips). Selling items and eggs to npcs can get you cash as well. Geogreymon egg sells for 100 , Kimeramon egg sells for 300, ice shards sell for 20 per 100 ice shards and black gears sell for 30. Some players will even pay more for eggs then they sell to the NPCs. Killing other players might make them drop some coins along with data, but don't expect anything on your first try.