Diffusion Stones, or 'Air Stones', are commonly used to aerate wort before fermentation, which helps ensure a healthy start to the fermentation process. Diffusion Stones can be attached to compressed oxygen tanks or air pumps (such as those used with aquariums). This "Stone" is made of stainless steel, so it's durable, easy to clean, and won't crumble in your wort after a few uses like other materials! The 1/2 Micron stone produces very small bubbles, which are ideal for efficient absorption of the gas into your wort. However, it takes more pressure to force air through the smaller pores. This is not a problem when using compressed oxygen, but lightly-powered air pumps may not work well. Long story short, use the 1/2 micron if you're using compressed O2 and 2 micron if you use an air pump.
The Diffusion Stone is also suitable for force carbonation of beer inside a keg, where the faster absorption of gas achieved by the small, ½ micron pore size is a great advantage.
It's important to clean and sanitize diffusion stones thoroughly before each use. And it helps to clean the stone immediately after use as well. Boiling the stone for at least 5 minutes in clean water is the only reliable way to clean and sanitize it completely. It's important to make sure the air being fed into the stone is clean, to prevent contaminants from clogging the stone or infecting the wort. Ensure a clean air source or install an air filter. Avoid touching the porous surface of the stone to prevent any of the pores from becoming clogged.
