Stellazzios are coins that are hidden in the game’s world, by which you can collect them and bring them to a NPC in Treno, named Queen Stella. Upon handing them over to her, she will reward you with Gil and various items. In total, there’s 12 Stellazzios hidden in Gaia, but upon giving them all to her, you will find out there is one more coin. She will hand you the 12 coins you already found, using the clues on the back of each, to hopefully find the final one. Find this one and bring it back to Queen Stella to finish the side quest. Note that none of the Stellazzios are missable, so you will be able to find them all on the final disc, if you happened to ignore them.
Found in the Village of Dali, inside of the windmill in the northern part of the village. Check behind the central pillar to find the coin.
You will find this coin behind a crate on the second screen of Burmecia.
Inside of Quan’s Dwelling, on the first screen, there is a rope you can climb down in the back. This leads to the bottom area, where there is a spring that replenishes HP and MP. South of this spring, if you search along the bottom edge, you will find the coin.
At the entrance to Treno, there is a fountain on the left side, where you can toss 10 Gil into it. Do this a total of 13 times to get this coin.
From the entrance in Treno, go up the stairs on the right side to arrive in the slums area of town. Climb down the ladder to find the item shop, but you will want to go behind it. Next to the one crate will be the coin.
You will find this coin inside of Black Mage Village, next to the beds inside of the inn.
On the second screen of Madain Sari, there is a fountain in the middle of the area. Go on the northern side of the fountain to find the coin.
When you return to Alexandria in Disc 3, take the boat to the castle grounds and head left from the entrance to the castle. You should be at the tower now, so go inside and enter the room on the left to arrive in an area with a floating statue. Check the upper right corner of this room to find the coin.
On your trip to Lindblum on Disc 3, head to the Business District and go north one screen. There will be some debris on the left side of the screen here and that’ll be where you find the coin.
You will find this coin in the town of Daguerreo, which is located on one of the islands in the southwestern part of the world map. Note you will need either the Sky Ability for Choco or an airship to reach the town. Once inside, in the very first room, go to the right side and in the bottom right corner will be the coin.
Once you reach Ipsen’s Castle on Disc 3, head inside the actual castle and in the very first room, you will find a chest on the right side, behind a pillar. That chest will contain the coin.
On Disc 4, once you obtain the Invincible airship and can go inside it, do just that to appear on the bridge. Go south one room and there will be a chest on the left-hand side. Open that chest for the coin.
You will not be able to get this coin until you have collected the other 12 coins and brought them to Queen Stella. Once you do this, she will mention that there’s another coin out there and will give back the other 12 coins, as there are clues on the back of them to pinpoint the location of the 13th coin. To avoid the complicated bit, the Ophiuchus coin will be located in the exact same spot as the Scorpio Stellazzio, in Quan’s Dwelling.
Hand this over to Queen Stella to receive the Hammer and finish the side quest.