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Any Pokemon that uses Sandstorm while holding a Smooth Rock will have its duration boosted to eight turns instead of the regular five.
Smooth Rock is absolutely vital if you wanted to get a sandstorm team going in the lower tiers. It is more or less useless in OU, where you have Tyranitar and Hippowdon, as Sand Stream users are available. Sadly, there are few sand strategies that are viable here, as most of the viable sand offense Pokemon are only available in the higher tiers, such as Sand Force Landorus and Sand Rush Stoutland, and in RU and lower, which are the only places you could use this item, you are basically limited to Sandslash and Gigalith. Hence, Smooth Rock is not a commonly used item, as there are far easier ways to pull of the same or a similar strategy with far more rewarding results.
Route 8 - Talk to the Parasol Lady standing just in front of the entrance to Tubeline Bridge between 5 pm and 9 pm.