There are currently two stones you can use when playing on the slot machines.
"All Mighty One"
"All Mighty Three"
What these stones do is give you a special spot in the middle column that lets you make more matches.
The single stone will give you one free spot, and the triple stone will give you three.
You can click on the suitcase underneath your Casino$ amount to see how many stones you have and choose which you want to use.
When you've selected the stone, you just play the slot machine like normal and the special spots will show up automatically ^^
Aside from randomly winning stones with your daily card pick, you can also buy them from this guy:
"All Mighty One" stones: 40 AmeG/100 uses
"All Mighty Three" stones: 100 AmeG/100 uses