Info: 500 g Bag
Zeolite rocks are natural minerals made of volcanic ash, silica and alumina, which have several unique properties making them very useful in a wide range of agricultural, industrial, and household applications. Not only do they have a negative charge -- so they're able to attract and exchange ions with nearby positively charged molecules, including noxious gases or chemicals in the soil or water like ammonia -- but they also have a microporous structure (a series of tiny caves within, as it were), so they're able to trap those substances within themselves. Even better, this isn't just a one-time operation: they're also able to release those chemicals, toxins, and musty odors again into salt water or into outdoor air under direct sunlight, so you can use them again many times.
Small rocks like these can be used to adsorb excess ammonia from aquariums or fish ponds; or as natural deodorizers near cat litter boxes, or in closets, lockers, or gym or hockey bags, for example - or even in the living room if you smoke the occasional cigar; or to absorb moisture and musty odors in the air from a bathroom, basement, linen closet, or cupboard under a sink. Some also report that they help reduce frost buildup in a freezer, so you won't have to defrost your fridge as often.
Zeolite (whose name literally means "a stone that boils") can be "recharged" by exposing it to heat (e.g., they can be baked in the oven, but that would just release the fumes back into your kitchen), or direct sunlight (especially in the summer), or soaking it in non-iodized salt water overnight. If they've really accumulated a lot, some suggest a combination: first soak it in a 5% non-iodized salt solution for 24 hours, then dump the water out, and spread the rocks on a cookie sheet, and allow it to dry in the sun for a day or even two.