Higromite - [alternate spelling hygromite]
Higromite had it origins in volcanic deposits colonized by micro organisms in prehistoric lakes. These volcanic deposits, combined with microbial activity produced a natural balance of trace minerals and organic elements.
Higromite is a silica based mineral rock. Silica is derived from quartz which is used to make glass, it is one of the most common elements in nature. A common misconception made is in associating silica with silicone and silicon valley which evokes visions of synthetic - plastique - unnatural and so forth. Silica is all natural. See: Hydroponic Silicon
There are claims parroted persistently on the web that " In side-by-side studies with other mediums, plants grown in higromite exhibited more compact growth, improved stem length, quicker establishment, better bloom breaking, improved leaf expansion, better head size with shorter inter-nodal spacing, and an increased overall yield." No one has able to furnish a source for this information or a link to any of the studies, and repeated searches on Google and google scholar failed to yield any relevant results.
Advantages of Higromite
1. Higromite does contain a high percentage of soluble silica which is vital for development of healthy root systems and as a consequence healthy plants.
2.Elevated Disease Resistance - Silica is not taken up by the plant directly, but gradually absorbed by plants as a result of microbial activity within the soil. The use of higromite will naturally facilitate the slow release of silica for uptake by the plant in a hydroponic system. Plants with access to ample silica have also been shown to have a higher disease resistance. Silica stimulates SYSTEMIC ACQUIRED RESISTANCE (SAR) in plants which increases their disease resistance. Note . 
However, in order for this process of uptake to occur, there must be sufficient nutrients already available to the plant and microbes that facilitate the uptake.
3. Higromite has ample moisture retention, not necessarily superior,but adequate. Oxygenation is enabled by the porosity of the rocks which allows more oxygen to reach the roots.
4. It is reusable.
Disadvantages of Higromite
1. Weight - Higromite is very heavy. Lifting and Moving it is like moving around buckets of rocks... Uhh actually it is buckets of heavy rocks.
2. Algae - they flourish with higromite, they thrive on the silicon dioxide and seem to be more of a problem over extended periods than with most other media. See Algae Buildup in Hydroponics
3. Some low grade versions and clones have reportedly lead to sludge build up in reservoirs and a rapid change in ph. This may be the result of silca rock being mistaken for higromite or mislabeled.
Other Grow Mediums