Roll20 system is quite flexible already, and they are continually working on providing more flexibility for the infinite types of games you can run on here. Welcome to the website. My advice is click the free button to start a Campaign and then click "Join Game" on your campaign, and enter it. Start to look around. From there you can ask more questions on the forum, or read the Wiki documentation linked at the top. Thirdly you might want to look under Looking For Game and apply to join someone else's games to experience other people's games, but since you have highly custom ideas maybe you would want to focus on developing/testing your own custom campaign first. The idea behind Roll20 is like trying to make, literally, a "Virtual Table Top", meaning they are trying to make an online version of your kitchen gaming table at home. It's meant to be like a blank table, that you can put whatever you want. Roll20 has dice, cards, handouts, journals, pictures/images, text chatting, voice talking, webcam visuals. You can use whichever features you want. You can put anything into those features that you can imagine. The gems, items, whatever you can dream of --- that's for you to type it into the text chat, or say it in the voice chat, or draw a picture on the screen, or upload a picture of it. Roll20 is meant not to support specific games or specific items in your game, but rather to be generic in a way that would support any tabletop game or any specific item, gem, ability, character, plot, storyline, environment, setting, genre, idea. I hope this helps give you the scope of what you can do here. Campaign is just a name for "a game", or "a blank table top space for you to put whatever you want". Don't think of a special meaning for campaign like thinking it means "D&D saga" or something. A campaign is what you can start for free on Roll20 to make yourself a blank open space to start putting your ideas on the tabletop and using the features of this site for your own customizations. The GM (Game Master) is the person who opens that campaign. The GM can evolve the items/equipment however you want. That's for the GM to tell the players, "Your gem turned into a holy fire kind of thing because you killed enough undead, congratulations." So yes, you can certainly do that here.
