From the Super Mario Wiki
Stones, also called Jewels, are valuable collectibles found in the game Luigi's Mansion. There are three different colors of Stones, each with their own name:
Ten of each color can be found in the Mansion, totaling 23,000,000G. To find them all, Luigi has to water all the plants he can find, examine a variety of objects such as buckets, gutters and gold chandeliers, catch Speedy Spirits, and light up all the rooms in the mansion except for the third and Basement floor hallways.