Remember the Barrels you sometimes encounter? When you attack them, sometimes they become monsters (they look like a giant chick stuffed inside the barrel). You can get Silver Gems from them as its Ultra-Rare drop.
Wanna get them (for a reason I dunno actually)? Then go to the Boulder Region, (blocked area above the Selman Grove, you can enter it after you are able to use the Wind Flowers), equip an Encounter Ring, and look for “I’m Stuck”. Here’s the info you’ll probably need:


Hope this helps. I’ll update more as I find more.
BTW, why do you need them?