Mycena Cave has two main types of currencies: Nuggets and Gems.
Nuggets are easier to obtain; they can be earned by posting on certain sub-forums*, playing games, selling and trading with other users, bank interest, or selling items obtained through Fishing from your inventory. Currently, you may earn up to a daily limit of 3,000 nuggets from playing games.
*Sub-forums that currently offer nuggets payout are: Welcome Lounge, Mycena Cave Discussions, Cave Stories, Other Stories, and Musings.
Gems are slightly harder to obtain; they can be earned through special events, the Gem Shop , and trading with other users. Gems are used solely for buying pets and their accompanying monthly items.
Unlike nuggets, gems can also be purchased with real money (USD) at the Cave Exchange .
This shop allows users to obtain gems without spending real money. Nuggets can be exchanged for gems at a rate of 3,600 nuggets per gem.
In addition to this official exchange rate, the shop also includes listings of any users who may be selling gems at a different exchange rate. Many users will offer discounted rates, so it is recommended to purchase from them to get the best deals.
A single Paw Print can be purchased for $5 USD in the custom shop. Paw Prints can be gifted both anonymously and with a name attached.
Paw Prints are attached to specific orders in the custom order queue and cannot be transferred from order to order. Should a player have too many Paw Prints by the time of their order or the order is cancelled, the excess PP are converted to gems at a 1 PP:4 gem ratio.
During the Spring Flower Rush! (held Feb. and March of 2014) event, flower petals were used as a currency in the Secret Garden Shop to buy event-exclusive items. They were obtained during the event by picking flowers, creating bouquets, and sending them to other users.
During the Summer Discovery Dig! (held in July of 2014) event, shinies were used as a currency in the Underground Marketplace to buy event-exclusive items. They were obtained during the event by making and sending words to other users.
During the Spring Swarm Extravaganza (held in April of 2015) event, tokens were used as a currency in Professor Mariel's Research Lab to buy event-exclusive items. They were obtained during the event by picking beads, creating pieces of jewelry, and sending them to other users.
Custom Credit can be used in place of USD when purchasing customs. They can even be used as partial payment for customs (i.e. if you have $40 dollars in Custom Credit and the custom costs $50 dollars, you would have a balance of only $10 dollars to pay). You can buy them for yourself or for another player, but are non-transferrable once they have been purchased. This means that they can only be used by whoever you buy them for. They can be purchased in increments of $5.
There is an upper limit to how much Custom Credit is allowed on the site. When the limit is hit, the Custom Credit shop is closed until the amount of Custom Credit on the site drops bellow 75% capacity. Additionally, an individual user can only have a maximum of $200 in Custom Credit. If someone were to purchase Custom Credit for a user who already has the maximum amount, the purchase will fail.