Now that two feet of snow in my area has brought just about everything to a grinding halt and I have finished shoveling snow, I've had some time to put this little essay together. I hope some of you find it useful and informative. I am a geologist by education and training and am licensed to practice geology in my state.
Finding "Flint" - a Geologist's Perspective
by
Half Axe
The following essay identifies some things to think about when searching for a suitable rock to act as a "flint" for striking sparks from a forged steel striker into a prepared tinder - a traditional method of fire starting used long before the advent of matches, butane lighters and ferrocerium rods.
These qualities make a rock a good one for flint and steel fire starting applications: it must be harder than the steel striker, or it won't be able to strike sparks from it; it must be tough, so that it will hold up through repeated striking of the steel; it must break in such a way that a sharp edge is produced to strike against the steel; it must be commonly occurring, so that it can be easily replaced as it is used up. Traditionally, the material which best exhibits all of these qualities is flint, a microcrystalline to cryptocrystalline, granular variety of solution-deposited quartz.
Hardness: the Mohs scale of hardness is a scale of relative mineral hardness developed by Austrian mineralogist Friedrich Mohs in 1824, who selected ten minerals for his scale. The minerals, in order of increasing hardness, are: 1. Talc; 2. Gypsum; 3. Calcite; 4. Fluorite; 5. Apatite; 6. Orthoclase; 7. Quartz; 8. Topaz; 9. Corundum; 10. Diamond. The Mohs test is a scratch test; if the test material will not scratch a mineral on the scale, it is softer than that mineral. A good quality steel file, such as would be used to sharpen an axe, has a Mohs hardness of 6.5. Such a file, even if worn down or broken, can be re-purposed to test rock specimens; if the file leaves a streak of metal on the rock, then the rock has potential for use as a "flint".
Tenacity: in geologic terms, tenacity is defined as a mineral's resistance to breaking, crushing, bending or tearing. As a general rule, rocks with a microcrystalline or cryptocrystalline texture have greater tenacity than other rocks. Granite, for example, does not make a good "flint" because its macrocrystalline texture causes it to break along crystal boundaries and the individual crystals end up being crushed. On the other end of the texture scale, obsidian has an amorphous texture which gives it a Mohs hardness of 5 and makes it too soft to be a good "flint".
Cleavage and Fracture: cleavage is the tendency of a mineral to break along its crystallographic planes (crystal faces), which reflects its crystal structure. Crystal faces are external expressions of weaknesses in the crystal structure; the atomic bonds are weaker perpendicular to the crystal face and stronger parallel to the crystal face. Rocks with well-defined cleavage do not make good "flints" because they break up rapidly with repeated striking of the steel. Fracture is the way a mineral breaks when it does not yield along cleavage; the atomic bond strength is, for all intents and purposes, equally strong in all directions. Of the four types of fracture, rocks which exhibit conchoidal fracture - a smooth, curved fracture which resembles the interior of a clam or oyster shell, commonly seen in glass or obsidian - have the best potential for use as a "flint".
Chalcedony, Chert and Flint: Much confusion exists regarding these terms in common use. In geologic terms, microcrystalline (crystals only recognized and distinguished under a standard microscope) and cryptocrystalline (crystals too small to be seen under a standard microscope, only distinguished using advanced methods) varieties of solution-deposited quartz divide into two categories: fibrous and granular. Chalcedony is the general term used for the fibrous (long, needle-like crystals) category, characterized by a waxy, translucent to transparent appearance. Common varieties of chalcedony are: carnelian (red); sard (brown); chrysoprase (apple-green); agate (alternating layers with different colors in concentric bands); petrified wood (wood has been replaced by agate); onyx (alternating layers in parallel planes); sardonyx (onyx with sard alternating with white or black layers); heliotrope or bloodstone (green with small red spots of jasper). The granular category is characterized by interlocking compact crystals of quartz smaller than 30 microns (a micron, or micrometer, defined as 0.001 mm or 0.000039 inch) in diameter, and a dull, opaque to slightly translucent appearance. The granular varieties are almost identical in texture and are differentiated only by color and depositional environment. Traditionally, chert exhibits white to variously light colors and occurs in bedded deposits, and flint is dark to black in color and occurs in nodules in chalk and soft limestone deposits. In geologic literature, however, the terms chert and flint are essentially synonymous and used interchangeably, chert being used to describe nodules and layered deposits in limestone and dolomite, and flint being referred to as a dark gray or black variety of chert. To add to the confusion, jasper is alternately defined as chert associated with iron ores and containing iron oxides which imparts a red color, or any red chert or chalcedony regardless of association with iron ores. Prase is a green variety similar to jasper, and occurs with it. Either category will, as a general rule, serve very well as a "flint".
Definitions used in the paragraphs above are adapted from American Geological Institute publications and Hurlbut and Klein's Manual of Minerology.
