Larimar is a rarity that becomes available at level 8. Like the other items from the "Rarities" section of storage, larimar doesn't occupy any storage slots.
There are multiple ways to get larimar including breaking up large stones, or as part of rewards for completing Merchant orders.
You can also find it when you explore islands, clear obstacles, and open gifts and chests.
Larimar will prove useful to you when it comes to fulfilling Dragôn Mysterium requests and upgrading the Stone Poem. You will also need it to craft jewels at the Loom.
If you need more help on finding resources in Elf Islands, check out our article Resource Sites!