I think pDragon is correct here; Opalescent White Pyramid Stones let you treat the keyed weapon as a martial weapon for the purposes of proficiency. Considering that means that other exotic weapons - such as the Orc Hornbow, for example - would also be treated as martial, as a DM I would rule that it lets you wield a Katana one-handed since the stone specifically lets you treat a non-Martial weapon as a Martial Weapon. IE, one-handed katanas. A normal, full priced stone would give you WP (Katana) which would fulfil the requirements for using it one handed.