I upgraded my Mega Ring and have been looking for Mega Stones, and I found a nice guide to the Mega Stone’s locations. However, the Mega Stone isn’t in the place it’s supposed to be, and I’m sure the location is right. Why isn’t my Mega Stone here?
There seems to be two factors for when Mega Stones appear:
You must have “upgraded” the Mega Ring postgame by battling your rival in Kiloude (after beating the Champion you get a pass to use the train in Lumoise to get here), then talking to professor Sycamore in Anistar City, this allows you to find Mega Stones not previously availble, in sparkling patches on the ground.
All Mega Stones found in this way must be obtained between 8 and 9 PM on your 3DS’s clock. Guides only note some stones as being “night only” but in my experience none of these sparkles would show up past 9 PM or before 8 PM. You can simply play with the system time if you cannot play in these hours or if you miss the critical time. Don’t worry if the game doesn’t look like it’s night time, nights are very bright in the pokemon world.