Final Fantasy XIV's Island Sanctuary may have a lot of available materials, but many of them can't be gathered immediately. Island Marble is one material you won't see at all when you're just starting to develop your island, but as you progress, it will become essential.
Island Marble is a material you'll end up needing for a lot of things, from upgrading your Workshops and Granaries to getting new tools for your mammet helpers. Without it, you're kept from expanding your available land, so it's very important to know where to get it when you need it. If you haven't reached the point in Island Sanctuary where you can acquire Island Marble, there are plenty of ways to speed up your progress.
Where to find Island Marble in FFXIV
First, you'll need to have reached at least Sanctuary Rank 13. Island Marble also requires the Islekeep's Steel Hammer; gathering from the correct nodes won't net you any marble otherwise. Once your rank is high enough and you have your tool, you'll be looking for Smooth White Rock nodes. While looking for them, keep in mind that they're more of a light brown, despite the name.
- Pristine Pond: The easiest place to get yourself some Island Marble is right by the entrance to The Wilds. Just a short distance north of the entrance and past the pond, you'll find Smooth White Rocks along the edges of the river. There are two clusters of these rocks in this area, making it the ideal spot to gather them quickly. To make sure you're getting them all, follow the river north for one cluster and look around where it branches west to find the other.

- Pirate Bay: Don't bother looking anywhere in the sand of this area; you'll find Smooth White Rocks a little higher up the hill (X: 15.2, Y: 14.5). The uneven terrain and grass make them harder to see, but you'll find them if you head south from the beach and look at the base of the short cliff. There are a few more located a bit further south, next to larger rocks.

Besides these locations, there are more places on the island to find Smooth White Rocks, but they're generally more spread out or difficult to spot. Overall, Pristine Pond will get you the most Island Marble for the least effort.