Hematite is a common iron oxide mineral (Fe2O3). Its appearance varies widely, ranging from deep reddish-brown (red ochre) to lustrous, silvery-gray (specular hematite). Regardless of its outer color, its distinctive feature is the reddish-brown streak it leaves when scraped across an unglazed porcelain plate. Geologists assess the material’s overall strength by considering its resistance to scratching, reaction to impact, and density, which collectively determine its durability.
Assessing Mineral Hardness (The Mohs Scale)
Mineral hardness measures a material’s resistance to scratching or abrasion, quantified using the Mohs scale. This ordinal, relative scale ranges from 1 (talc) to 10 (diamond). Hematite typically registers a hardness value between 5.5 and 6.5. This moderate range means it is harder than common window glass or a standard steel pocket knife, which hover around 5.5.
Hematite is considerably softer than many common minerals, such as quartz, which has a hardness of 7. Any material with a higher Mohs value can easily scratch hematite, which impacts its use as a polished stone. The variation in its hardness rating accounts for the different forms it takes, from soft, earthy varieties to harder, crystalline specimens.
Tenacity and Resistance to Breaking
While hardness measures resistance to scratching, tenacity describes a mineral’s response to mechanical stress, such as crushing or impact. Hematite is characterized as brittle, meaning it tends to shatter or crumble when struck with force, rather than deforming or bending. This brittleness is a significant limiting factor in its overall durability, as a hard strike can cause the material to fracture completely.
The mineral structure lacks true cleavage, which is the tendency to break along flat, predictable planes of weakness. Instead, hematite exhibits an irregular, uneven, or sub-conchoidal fracture pattern. This irregular breakage results in shell-like or jagged fragments when the mineral succumbs to impact stress.
Density and Specific Gravity
Hematite is noticeably heavy for its size, a trait related to its high Specific Gravity (SG) and iron content. Specific gravity is a unitless measure that compares the density of a substance to the density of water. Hematite possesses an SG typically ranging from 4.9 to 5.3.
This value is exceptionally high compared to many common rock-forming minerals like quartz, which has an SG of approximately 2.65. The substantial weight results from the mineral’s high concentration of iron, which can constitute up to 70% of its composition.
Practical Durability in Use
The combination of moderate hardness, brittle tenacity, and high density dictates hematite’s practical durability across its various applications. Its primary industrial function is as the world’s most important ore of iron, where its density and iron content are the most valuable properties. High density also makes it useful as a weighting agent in drilling fluids, increasing the fluid’s mass.
When cut and polished for jewelry, hematite’s durability is more constrained. Its mid-range hardness (5.5 to 6.5) means jewelry can easily be scratched by harder stones, such as topaz or granite countertops. Furthermore, its inherent brittleness makes thin pieces or rings susceptible to chipping or outright shattering if dropped onto a hard floor.
To maintain the mirror-like metallic luster of polished hematite, it should be stored separately from other jewelry to prevent abrasion. Due to its weaknesses, hematite rings and bracelets should be worn with caution and removed during strenuous activity.