There are plenty of things to do in Final Fantasy XIV's Island Sanctuaries, and one of the things players will do very often is go to gather materials in the Wilds. Luckily, while players grind their way to their next Island Sanctuary Rank, they'll also collect some shiny new gems to trade in for rewards, called Islander's Cowries.
There aren't many ways to get Islander's Cowries, and farming them can be quite the grind. But once you've got enough of them, they can be traded in for some pretty sweet rewards that can also make you a bunch of gil if you've got everything you need already. Islander's Cowries are one of three currencies in FFXIV's Island Sanctuary. They are green in color and can be spent at the Horrendous Harder for a few different things.
Islander's Cowries aren't really meant to be something to farm. The rewards are tempered, because there is technically nothing stopping players from gathering up a bunch of stuff in the Island Sanctuary and then selling it to the Enterprising Exporter for the currency.
Seafarer's Cowries and Felicitous Tokens are the two currencies that are most sought out in the Island Sanctuary, because they have rewards such as minions, mounts, and fashion accessories. Islander's Cowries have far less desirable rewards, only offering some furniture items, materia, and party items that players don't typically care for.
It's not the easiest way to farm for materia either, making the Islander's Cowries something that most players don't tend to farm, but it is a non-combat way to get materia and can be spent if you happen to collect a bunch of it. In case you do need it, here's how to farm for Islander's Cowries.
How to farm Islander's Cowries in FFXIV
The method to farming Islander's Cowries is fairly simple. Fly out to the Wilds (the area where you can gather resources and materials) and start gathering. There are several areas where players can gather quickly, including the dirt area at X: 22 Y: 26, which players can make quick rounds with.

There are also some later parts of the game that are quick for gathering things, but it will require higher levels of Island Sanctuary Rank, and to get the most out of them, you'll want the highest rank tools you can get.
The first area is in the Mountain Hollow, by the Shiny Spring point-of-interest at X: 23.7 Y: 19.2. Starting at the Glowing Fungus at those coordinates, I tested how long it would take to make an entire round of the cave. I gathered 15 nodes in about a minute, keeping sprint up at all times. Most nodes gave three materials with a fully upgraded toolset, but if your Island Rank prevents you from making high-end tools, you might get less from each hit.

If you need things like Island Crystal Formations or Island Hawk Eye Sand, you can head to the Mother Lode point-of-interest, and that little circle will be just enough for you to continually run around without having to leave the area to spawn in a new node. This area holds 11 nodes and takes about 45 seconds to clear. If you're purely looking for Islander's Cowries, the Mountain Hollow area is best, since it gives three materials per hit, while other areas give far less.
As long as you're in an area with more than 10 nodes in it, you'll be able to farm up materials very quickly, since you won't have to stray too far to get a node to respawn. The areas above are just a few of the places you could go to gather materials, but if you're out in the Wilds farming for something you need, it can be worth it to farm what's around it as well to both get the Islander's Cowries and to do less running around by getting the nodes you need to respawn faster.
Where to get Islander's Cowries
Once you've gathered a bunch of extra materials, you can go back to the Island's Cozy Cabin and talk to the Enterprising Exporter NPC to trade in all of your extra materials. The easiest way to do that is to set the amount you'd like to keep in the lower left corner, and it will automatically convert all of your extra materials into Islander's Cowries. Otherwise, you can select each individual item and sell as many as you'd like.

The items themselves are capped at 999, so make sure that you're constantly trading in your extra materials if you end up with a lot of the same thing.
What to buy with Islander's Cowries
Because Square Enix knows that there are no shortage of players that will grind content to make money, they've tempered the rewards for Islander's Cowries, since the materials in the Wilds is able to be farmed without any kind of restraint to how often or how many items they can gather at any given time.
Since just about anything you gather in the Wild can be sold at the Export NPC, the rewards are fairly expensive compared to other methods to obtain these items. When you're ready to cash in your Cowries, head to the Horrendous Hoarder and select "Purchase items with islander's cowries". This will bring up a menu with some outdoor and indoor furniture items, party items, and materia.
None of the furniture items bought with these Cowries is worth anything on the market, so your best bet if you have extra Islander's Cowries is to purchase materia with it. A IX materia costs 1,750 Cowries, and a X costs 3,500. This is higher in cost than the Seafarer's Cowries which cost 750 for a IX and 1,500 for a X, because it isn't possible to endlessly farm that currency. For those who have a gathering class they use often, this is also a great currency to use on Hi-Cordials.