The category of natural grinding resources comprises the grit materials quartz (flint), corundum, emery, garnet and diamond. Except for diamond, these materials are usually of insufficient strength, so they are of secondary importance in comparison with synthetic grit materials for most industrial purposes. A further reason for the exclusion of natural grit materials is the insufficiently manageable and non-reproducible grit quality of natural products. The only exception to this rule is the diamond, which is still used today in industrial grinding technology.
Quartz
Quartz includes, among others, flint and tripolite. They are composed mostly of SiO2 and reveal impurities in the form of iron oxide FeO and titanium dioxide TiO2. Flint, a comparatively soft grit material (820 HK) fracturing in no sharp edges, is primarily used in the wood and leather processing industry. Tripolite is used for the polish-grinding of metals [COLL88, SPEN70].
Corundum and Emery
The natural grit materials corundum and emery consist predominately of macrocrystalline aluminium oxide (Al2O3), the content of which determines its cutting properties to a decisive extent. Natural corundum has an Al2O3 content of 80 to 95 %, attains a high level of hardness (2050 HK) compared with other natural grit materials and is sufficiently tough. Emery is composed of up to 60 % Al2O3 and contains iron oxide Fe2O3 as an additional component. For this reason, emery is less effective than corundum. Today natural corundum and emery are rarely used, since the two minerals crack in both round and block shapes and form few sharp cutting edges, they are now seldom used. However, they do still find practical application in rubber-bonded grinding components and emery paper. Furthermore, they are also utilised in unbonded form for polishing optical glasses [COLL88].
Garnet
In the case of natural garnet, both its hardness (1360 HK) and its shell-shaped fracture, which causes the formation of numerous new cutting edges, are decisive. Garnet is predominately utilised for abrasive paper and cloth for the treatment of hard and soft woods, as well as in unbonded form in the polishing treatment of optical components [COLL80].