This strange rock dramatically increases the weight of all objects on it or near it.

    Effect: Loadstone squares are difficult terrain. A ranged attack that passes through a loadstone square takes a -2 penalty to the attack roll.
    Usage: Use loadstone areas improve the changes of brutes against characters with ranged attacks, forcing those characters to maneuver for better line of effect. This terrain is useful for giving extra protection to soldier or brute creatures guarding a narrow point, such as a bridge across a chasm, from ranged attacks while they engage melee-based characters at the choke point.

Published in Dungeon Master's Guide, page(s) 68.