Infused stone is stone infused with Vis. Destroying one drops a shard of its corresponding type (Air Infused Stone drops Air Shard, Fire Infused Stone drops Fire Shard, etc.). It also seems that certain shards are more common than other ones (Fire Shards are more common in deserts, Water Shards in rivers and oceans, Earth Shards in mountains)
The types of Infused Stone (and shards) are as follows:
Air, Fire, Water, Earth, Vis, Dull
The Shards from Infused Stone are highly valued for their number of aspects, making them valuable for Research. In addition, they are used in quite a few recipes.
Here is a video that shows you how infused stone works http://www.youtube.com/watch?v=Me9Y6QTML58
On occasion there is so much magical energy in an aura that it would permeate solid stone to form Infused Ore. The magic forms crystals within the rock that can be harvested.
These crystals have various elemental properties and have many uses in Thaumaturgy.
It should be noted however that it is not always a good idea to harvest all the infused ore you come across for it has another useful property. If the local aura dips low enough the crystals will turn back into magical energy that replenish the aura, leaving depleted ore behind. Should the aura ever rise high enough, depleted ore may turn back into infused ore. It has also been known to happen for normal stone to spontaneously turn into infused ore, but only if the local aura reaches extremely high levels.
Dull Infused Stone
Infused stone will be turned to Dull Infused Stone (grayish looking one on the far right in picture) If a nearby Aura Node gets a high enough energy level, the Dull Infused Stone can be recharged and turned into Infused Stone. Also, according to the Thaumonomicon, stone can apparently be turned into Infused Stone if the local aura is high enough and maintains such a state for a long period of time.