Almost everything at Mycena Cave is bought with our in-game currency, nuggets. These can be made via the games, by posting in some forums, through bank interest or by trade with other players.
Our secondary currency, the gem, is used solely for buying pets and their accompanying monthly items. Gems can be aquired from the Cave Exchange and can be bought with nuggets or dollars. The cost of one gem is 3,600 nuggets per gem, but you can probably get a better deal buying gems from other players. Alternatively, you can buy one gem for $1
Nothing that is sold for nuggets will be sold for gems, and vice versa (by Mycena Cave anyway - you can do what you want when trading amongst yourselves). You also cannot buy nuggets for real money - the closest equivalent would be to buy gems and sell them for nuggets. Gems don't earn interest, but if you sell your gems for nuggets and then bank them, you can earn interest and use the nuggets to buy gems again later.
For the most part, we leave how you use the economy up to you. The only caveat is that you may not sell anything on this site (directly or indirectly) for real money. This includes selling or trading things on this site for things that you can sell for real money.