Luckstone

A luckstone is a gray stone that modifies a player's Luck and Luck timeout. A noncursed luckstones adds three Luck (which may exceed the ordinary maximum of ten) and a cursed luckstone subtracts three Luck.

A cursed luckstone prevents the timeout of negative Luck, blessed prevents the timeout of positive Luck, and uncursed prevents the timeout of any Luck. Luck is a scalar (i.e. single) value, which means blessing your luckstone is not terribly necessary unless your total Luck is negative (though it's useful in that it's harder to curse a blessed luckstone than an uncursed one).

Multiple luckstones have no cumulative effect, except that having more cursed luckstones than blessed will give you the effects of holding just one luckstone, which is cursed, and so on. A backup luckstone (carried in a container or left in a stash) may be useful in case your primary luckstone becomes cursed (and uncursing equipment is limited).

There is a guaranteed luckstone in each of the three variants of Mine's End.

The Heart of Ahriman is a luckstone, which allows you to determine whether any gray stone is a luckstone through abuse of the naming artifacts trick.

The Tsurugi of Muramasa and The Orb of Fate both act as luckstones.