Alex,
That's not entirely right.
There are two kinds of silica used in these applications. One is referred to as molecular sieve or zeolite silica and is used in chemistry. This type is identified by the fact that it does not look like transparent or translucent crystals but rather little clay pellets. We used them in the lab to achieve selective adsorption of various molecules including water. It's also used as a catalyst substrate. Zeolite is sometimes used in those dessicant packs in various moisture sensitive goods to maintain dryness.
The type that is used for kitty litter as well as the bead-type humidity regulation substance is buffered silica gel. Buffered silica gel commonly comes in the form of chunks of crystals or spheerical beads. Buffered silica gel, once conditioned upon exposure to a stable humidity will tend to retard movement of the environment from the conditioned humidty level. That is how you are able to buy 60%, 65% and 70% beads. Same material. Conditioned differently.
Because of the low moisture content of silica gels, they do not have a bountiful amount of moisture to give up if say, you leave your humi open. They will dry out. However, in a well-sealed container, they are quite effective.
So when you cut open a dessicator package, if the filling looks like opaque pellets, they are zeolite. If they look like crystals, they are silica gel, although probably not of the proper functionality to maintain moisture in a humidor.
Wilkey