1) Travel to Verdanturf Town
2) Talk to the little girl in the bottom-left corner (right below 03 & 04 in the picture)
3) She will say she can't find her Pokémon. You need to click on the sign in the middle of Verdanturf
4) A Shroomish will appear behind the sign. The girl and Shroomish will be reunited
5) The girl will give you the Intriguing Stone
6) Travel to Rustboro City
7) Head to the Devon Corporation (labeled in picture)
8) Go the top and talk to Mr. Stone, the president of the Devon Corporation.
9) He will notice the Intriguing Stone and identify it as a Pidgeotite. You will then receive it.