Gemstones are very well hidden items in Luigi's Mansion 2. There are five different types for each mansion;
Amethysts are in the Gloomy Manor. Emeralds appear in the Haunted Towers. Rubies are in the Old Clockworks. Sapphires are in the Secret Mine, and diamonds are in the Treacherous Mansion. Each is hidden very well, and the player must have a keen eye to spot clues to where they are. For each mansion there are 13 different gemstones, each with a different shape. Collecting all the gems will unlock a special statue of Luigi for the player to admire. It is decorated with the gems that were collected.
Check the suit of armor on the right to make it's head fall off. Suck up the helmet and shoot it at the painting of a knight head. It will open up and make a gem appear.
Open the fridge to reveal the gem encased in ice. Then check the furnace to start a fire. Suck up the ice block and move it over to the fire, freeing the gem.
Star
A-3, A-4, A-5 and Bonus
Foyer
In the Patio, go left and use the Dark-Light Device on the wall, revealing a door to Under the Stairs. Go though the next door to end up on the other side of the fish tank, allowing access to collect the gem.
Reveal some pipes using the Dark-Light on the stairwell of the Tower Lobby, and then reveal a door on the left side of F1 in the Tower Lobby. Use the hitchhiker plant to kill the Carnivorous Plant. Open the chest.
Use the cobwebs to burn your way through the left tunnel. Take the bucket from the left and fill it with water. Then go right to water the plant. Use the Strobulb for your reward.
Star
B-3, B-4, B-5, and Bonus
Old Cemetery
Take it down from the tree branch it sits upon, just before the main part of the graveyard.
Go across the boxes on the right side of the bottom part of the room, revealing a bridge along the way. Go into the hidden door and complete the challenge to get a gem.
Look through the binoculars at the entrance to the Terminal. Look for the dark cave and you'll see a bat go from the cave and drop the gem into a barrel. Go collect it afterwards
Outside to the left of the entrance is a stove, with a chicken on top. Light the stove and use the flaming chicken to melt the ice around a painting outside. Use the Dark-Light on the painting, and collect the gem.
On the lower left wall is an axe that is hidden with Spirit Balls. Uncover it with the Dark-Light (there is a hidden arrow pointing to it in the ice). Use the Poltergust 5000 on it to find two logs. Put both on the ice to break it, and collect the gem that was inside.
While sliding down be prepared to go to the left. There will be a big spinning wheel blocking a path to a gem. The player must slide through one of the holes in the wheel to get it. This may take a couple of tries.
Light a piece of coal, and look up. There should be a gem encased in ice above. Shoot the coal at the ice to melt it and ma,e the gem fall to the ground.
Go to the left side and find the missing mine cart with the Dark-Light. Quickly go over to the right side to capture the Spirit Balls. Return to the left side to find the gem hidden in the snow.
Go to the very bottom of the mine, in the Crystal Quarry. In that room there should be a portal that can be activated with the Strobulb. Go though and you'll end up in the Chalet. Luigi will land on a chair, knocking the gem from it's hiding spot.
After the Underground Lab, follow the direction the suits of armor point shen checking them. At the last tunnel with no suits of armor, go to the farthest back point and go right. Use the Dark-Light on the painting of a spider web and light it with the fire nearby. Burn the webs you saw along the way for a gem.
From the Underground Lab door go right, down, right, and up twice. This will lead you to a door that goes to the Cliffside. Take the gem down from the vine.
Pull on the mammoth trunk in the Ice Age Exhibit to access the Gargoyle Roof. Find three stars around the "room" and shoot them into the giant star made of smaller stars. A gem will fall from the sky upon completion.
Water the spout close to the door, and use the Strobulb on the plant that appears. A gem will pop out. You can find the bucket in the Dark Age Exhibit and the water in the Jungle Exhibit. Use portals to traverse.
Place something like a log or a bucket of water of the life savior on the ship to make the wheel go down. Go out to the end of the ship and open the chest with a gem in it.
Using a Balloon Berry, fly up and right to find a gargoyle with a blue book. Use the Strobulb on it to make it drop the book and fly back down. Put that book into the bookshelf in the Dark Age Exhibit.
Either use a Balloon Berry or enter from the East Balcony to get to the top part of the Jungle Exhibit. Find the Golden Balloon Berry and collect all the coins for ca chest with a gem in it.
Use a Balloon Berry to fly to the tiop left portion of the Ice Age Exhibit. Collect all the coins from the Golden Balloon Berry for a chest with a gem in it.