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|On Route 18, in the grass directly under the connector leading to Bicycle Road.
From the Fuchsia City Pokémon Center, jump down the ledge to your right and walk up.
Get on the yellow path and follow it left.
Continue following the path left even when you exit the city and enter Route 18.
You’ll find the Altarianite directly South of the connector, hidden in the top-left corner of the tall grass.