Garnet is not a single mineral. Rather, it is a group of minerals that share a common crystalline structure and a similar chemical composition. The most common varieties of commercially available garnet are Almandite (or Almandine) and Andradite. Although Almandite is the heaviest and hardest and Andradite is the lightest and softest of the garnet family, both are relatively hard and relatively heavy.
A more important attribute not necessarily associated with the chemistry is how the garnet was formed. Because of their hard rock deposit formation, all of the deposits of Andradite and some of the Almandite garnets are structurally weaker than full crystal alluvial Almandite garnets. In hard rock garnet deposits, many crystals can make up a particular grain size. A high number of crystals in each grain means the grains are more friable or break down more easily. This tendency to break down results in a greater level of finer particles of dust, slightly lower performance in a one-time use, and a reduced ability to be recycled. Usually, the lower performance is partially offset by the lower price of the Andradite compared to the Almandite.
A very simple friability test will go a long way in understanding how a particular garnet will break down and generate dust.
Place about 10 grains of each type of garnet you are considering on a flat, hard surface.
By using a quarter and the pressure of your thumb, try to break each grain of garnet individually, if possible.
The easier it is to break the grain, the higher the dust levels will be and the fewer times you can recycle the garnet. For recycling projects, choose the sample that is hardest to break.
It is imperative that the end user (contractor) acquire samples of each garnet product under consideration. At most, a quarter of a pound of each is needed. The friability test should be performed before ordering a particular type of garnet. When the selected product is delivered to the job site, the test should be done on the product at the site to see if it is comparable to the sample. Because everyone exerts different pressure, the test gives only a relative indication of friability.
All garnets do not yield the same profile, cutting speed, and recycling ability.
The harder, full crystal garnets (Al-mandite) will generally yield faster cutting speeds and lower dust levels than the more friable, fractured garnets (Andradite).