Bejeweled Monsters are a rare version of a monster, they have a jeweled texture and carry a treasure over their heads.

There are 8 bejeweled monsters in Dragon Quest Treasures and you will need them to get two achievements "Sparkly Spotters" and "Crazed Completionists"

Crystal Objects can only be obtained by killing or stealing from Bejeweled Monsters.


TIP FOR KILLING/RECRUITING
Always carry some Stun pellets and Buddy pellets, this way when you find a bejeweled monster you can instantly stun It before It runs.

Note: Bejeweled monsters are not the same as metal monsters

Sometimes when a group of monsters return from a dispatch mission they will give you the location of a Bejeweled Monster.
Those quests are the best way to find those monsters since there is no luck involved and you can save before the fight, so if something goes wrong just reload.

While trying to find Bejeweled Monsters learning about the "camera trick" is really helpful.

To briefly explain how It works, in this game monsters in the distance only exist when the camera is in their direction, so if you get a bit farther you can keep spinning the camera until a Bejeweled version of the monster spawn.

You can easily find Jade Slimes in the south side of Wingswept Moors near a big tree, get to the cliff behind the tree and spin the camera. Sometimes you will get an Emerald King Slime.

The Amber dracky can be found in Cinderback Ridge, north part of Claret Knoll. Near the fireplace get to the edge of the plateau and spin the camera.

The Ruby golem can be found in Maneland, south part of Bonedry Dunes. The golem spawns near the sand river, get on the upper part and spin the camera.
This one is a bit easier during the day.

You can find the Emerald King Slime in the same place as the Jade Slime, but It is a bit harder.

There is another spot nearby that is easier.

You can find the Onexterminator in the north part of the hinterquarters, next to the station.

This one is close to the Aqua Marine, so you can try and get both on the same trip.

Pearly wight can be found on Cinderback Ridge, north part of Lumbar Lavaland. Get to the highground near the cactus and spin the camera.

There are a lot of Lethal Armour in Cinderback Ridge, so It is kinda easy to find, there is a spot near Bloodleaf Bosque station that you can use the camera trick to spawn one.

I believe there is only one spot where Ultra marine spawns, It is in the north part of Hinterquarters.
There is a small spot where you can get on and rotate the camera until the Loss Leader turns into the Ultra marine.

I got most of my info from the dragon quest wiki, I just took the screenshots and tried to explain in an easier way what to do.

Check the link if you want more info.

https://dragon-quest.org/wiki/Category:Bejeweled_monsters