What They Are
Vis shards, also known as Vis crystals when in the world, are both an "ore" and a crafting component used in many magical creations. Although being classified as an ore, Vis crystals are not situated in stone, instead actually growing on it and spreading if the Aura is high enough. Vis crystals begin to grow when the Aura is so saturated with Vis that it settles into the stone itself, growing in a tangible form. These crystals grow in clusters as small as 1 and as large as 4, and can expand or shrink as the Aura around it changes. One can think of the clusters of crystals as almost a form of battery, being "charged" and storing Vis when the Aura is high, and being "drained" and releasing Vis when the Aura is low. As implied before, clusters of Ccystals can be released back into the Aura if it dips to low in the given aspect, but at least one crystal in each cluster will remain.
Where to Find Them
One can find natural Vis crystals growing in the world, or, after researching Crystal Farmer in the Basic Information tab, "plant" and "grow" them. Vis crystals can only be harvested as a whole at first, meaning that all must be taken at once or none can be acquired. However, upon researching Crystal Farmer, it is possible to take one at a time from clusters, allowing for more sustainable gathering of crystals.
Although there are 8 different types of Vis shards, only 7 can be found naturally, with the Balanced Vis shard, being found below, having to be created via Alchemy. One can find Vis crystals of all 6 Primal aspects growing, but because Taint also exists in the Aura, Tainted crystals can grow, and be planted and cultivated, as well. However, because Taint shards can be hard to obtain, one can create them via Alchemy after Taint shards or crystals have been scanned and the proper research unlocked. Finding a patch of Vis crystals is usually relatively easy, as they emit natural light depending on the number of crystals growing in the cluster, with only a light level of one being released with 1 crystal and a light level of 11 being released with 4 crystals.
Balanced Vis Shards
Balanced Vis shards are the only Vis shard that does not naturally form in the world and must be created Alchemically as so. They have many uses in almost all areas of magic, making them extremely useful, if not slightly annoying to create. However, to assist those new to Thaumcraft, there is a suggested recipe below.
The easiest way to create Balanced Vis shards is to create them 2 at a time using a piece of gunpowder, 2 pieces of andesite, granite, or diorite and 4 pieces of sugar cane using 2 Ordo shards as the catalyst. After one has researched Alchemical Manufacture, they can also add 4 dirt after the Shards have been created to use up the excess Herba aspects in the Crucible. If one follows that recipe, they can create Balanced Vis shards using almost entirely renewable or otherwise plentiful materials without any waste. The grass created can be placed down and broken to be used again as dirt. However, care should be taken, as if one tries to create too many at one time and place more than a single piece of gunpowder in after Alchemical Duplication has been researched, as it will remove some of the Ignis and Perditio aspects from the Crucible, creating 2 pieces of gunpowder in return, and potentially ruin a recipe. A fix to this can be smelting down gunpowder into Essentia and sending it through an Essentia Crystallizer and using the crystallized aspects instead. This allows for more to be created at once more quickly, but can result in the loss of some aspects, as Essentia smelting can be inefficient and also release Flux into the Aura. However, the recipe described is only one recommended recipe, and one can create Balanced Vis shards using any of the 6 Primal shards and their missing aspects.
Salis Mundus, referred to commonly, according the the Thaumonomicon, as Thaumaturge's Salt, has many uses in crafting recipes and Infusion, just as the Balanced Vis Shard from which it is made. It is used as a binding agent to hold different aspects together, as it is comprised of all 6 Primal aspects like the Balanced Vis Shard it is created from. To create Salis Mundus, one must simply smelt a Balanced Vis Shard in a furnace or an Infernal Furnace, although it does not matter which is used as the Infernal Furnace cannot offer any extra items for smelting Balanced Vis shards. Salis Mundus can be a good source of Vitium Essentia early in the game, when venturing to Tainted Lands can be dangerous and, even more so, difficult to find, unless produced.