Matt Bird writes all sorts of nonsense, but he dedicates a large chunk of his time to writing game walkthroughs.
(Note: This part is entirely optional. Skip ahead if you don't care about these awesome items.)
Throughout this walkthrough, we've come across about a billion and one treasure chests, and among them are a load of chests Oliver has been unable to open with his little Spring Lock spell. There's a good reason for that: his wand wasn't strong enough. Now that he has a full-power Mornstar, though, he can open any chest he pleases. That means it's time to meander back to all the spots we've previously visited and get all those juicy purple chests that were sadly bypassed. Here we go!
Ding Dong Dell
The purple chest in the city is on top of a building across the street from Swift Solutions. Inside you'll find a Set of Shining Scales.
Ding Dong Well
The chest of Ding Dong Well is near the end of the well. Make your way to the lower section of the well and check the western passage. At the end, in the right niche, is a Set of Blackwyrm Claws.
(You can also use this opportunity to open the green chest to the south of the second waystone. Inside is a Water Bomb Gem.)
Enter Golden Grove from the southern entrance and make your way to the waystone. The chest is beside it, and it contains a Hocus Focus Gem.
(There's another green chest to get here, on the first set of mushrooms from the northern entrance. Fire from the third mushroom down to get a Bar of Fairy Chocolate.)
Take the first right from the entrance and follow the edge of the path to the east and south. Hidden at the top of a small rise is the chest, which contains a Spire of Fire Gem.
Head to the Black Market building where you got your guard outfits when you first came to Hamelin. Inside is a chest with a Tricksy Nixy Gem.
Make your way up the trail to the cabin. Just west of the cabin, beside a tree, is a chest containing a Dark Cloud Gem.
Alchemist Brothers Forest
The forest is to the east of the Vault of Tears, atop some cliffs. It contains a Night Blade.
The Vault of Tears
Run through the Vault to the room that was originally filled with water, in the northwest of the dungeon. The chest is in a pool of water on the lower floor near the east wall. Inside the chest is a Steam Hammer.
The Glittering Grotto
The chest is in a large room adjacent to where you fought Cerboreas. Find your way to the second waystone and check the wall to the west for a hidden passage. This leads to the chest. Inside is a Helm of Hamelin.
The final purple chest is in the house to the west of the palace. Inside is a Bottle of Nostrum.
Not a bad haul, eh? With all that collected, it's time to forget the side quests and get down to business. Oliver's got a Clarion to construct!