While no crystal can directly "absorb" water vapor from the air in the same way a sponge absorbs water, some crystals can attract and hold water molecules through a process called hygroscopy.

Here are some examples of hygroscopic crystals:

* Sodium chloride (NaCl): Table salt is slightly hygroscopic, meaning it attracts water molecules from the air, especially in humid environments. This is why salt can become clumpy over time.

* Calcium chloride (CaCl2): This is a highly hygroscopic salt often used as a desiccant (drying agent). It absorbs water from the air, making it useful for drying out spaces or materials.

* Potassium hydroxide (KOH): This is a strong base that is very hygroscopic and can even absorb enough water to dissolve itself.

* Magnesium sulfate (MgSO4): This compound, commonly found in Epsom salts, is hygroscopic and often used in drying applications.

Important Note: While these crystals attract and hold water molecules, they don't literally "absorb" them in the same way a sponge does. The water molecules are more like attracted and held onto the crystal's surface.

Further Reading:

* Hygroscopic: [https://en.wikipedia.org/wiki/Hygroscopy](https://en.wikipedia.org/wiki/Hygroscopy)

* Desiccants: [https://en.wikipedia.org/wiki/Desiccant](https://en.wikipedia.org/wiki/Desiccant)