Use Wear:
Ground Stones - Quartzite
Raw Material
The preferred rock for grinding stones in Tigrai is a slightly friable quartzite. However, some households may not have access to the best quarry sites so may use a quartzitic sandstone.
Like other metamorphic rocks, quartzite falls along a compositional and structural spectrum from less to more ‘metamorphized’. A true ‘quartzite’ is composed almost entirely of recrystallized, interlocking quartz grains. During metamorphism, a protolith quartz-rich sandstone is subjected to heat and pressure by geologic processes. This causes quartz to recrystallize into larger grains that interlock with each other (Figures 1, 2). The resulting strong bond created by the interlocking produces a more stable and robust fabric within the rock.
The preferred stone used in Tigrai falls on the spectrum of quartzite where metamorphism has not reached complete recrystallization. Ideal grinding stones are closer to that phase than the other end of the spectrum, where quartz grains in sandstones are still primarily cemented with silica or carbonates and do not interlock with one another. Quartzitic sandstone (Figure 2) has not been affected by as much heat and pressure as true quartzite. The quartz crystals are smaller, and there is still sand (silica) bonding the crystals, but the rock overall is weaker than quartzite and will shed sand when used as grinding stones. This type of stone may be used by households who do not have access to high quality quarries or did not have the labour force available to move the boulders from the better quarries to their home.
Stones without interlocking crystals (e.g., sandstone) break along grain boundaries and produce ‘sand’ while grinding, an unfavorable characteristic according to interviewed Tigrayan consultants (Nixon-Darcus 2014:83; Nixon-Darcus and Meresa 2020:23). The sand mixes with the flour and some of the smallest particles are not removed during sieving, ending up in breads and other foods made with the flour. The preferred quartzite for grinding stones is that which fractures less frequently along crystal boundaries but instead primarily break across crystal grains, producing sharp, conchoidal fractures. The amount of sand produced during grinding is typically negatively correlated with the amount of recrystallization in the quartzitic rock. Conchoidal fractures of larger quartz grains provide coarse grinding surface textures preferred for grinding large size cereal grains such as wheat, barley and sorghum. For the smoother surfaces used to grind small size cereal grains t’ef and finger millet, the preferred quartzite offers a levelled surface with little to no spaces between the interlocking quartz crystals. The small grains therefore do not “fall through the cracks”.
The quartz crystals appear clear, translucent with some colour (white, pink most common), or opaque (Figures 3-5). In its natural state, quartzite matrix from Tigrai is made up of rounded (Figure 6) or angular quartz crystals (Figure 7), or a combination thereof, as well as various relative abundances of minor minerals. In some instance, quartz crystals were so compressed together through metamorphosis that the shapes of specific crystals are difficult to identify. Rounded crystals in sandstone protoliths are a result of physical weathering caused by sediment transport (i.e., rounded sediment grains have travelled far from their primary geologic context and thus are more ‘mature’). Naturally occurring angular crystals are much more common in the samples analyzed with 70% (14 of 20) having greater than 90% angular compared to 10% rounded. The angular ratio was as high as approximately 99% in 45% (9 of 20) of the samples.
*DinoliteTM magnifications are indicated in parentheses at the end of the Figure descriptions
For references cited related to this project, please click here.