Flint
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It’s not just minerals like agate, jasper, carnelian, and chrysoprase that are made of cryptocrystalline
| Formula | Group or Type | Shape | Hardness | Specific Gravity | Streak | Luster |
|---|---|---|---|---|---|---|
| — | — | — | 6.5–7.0 | 2.5–2.8 | — | — |
For this type of crystallization, we don’t mean “crypt-ocrystalline,” like crystalline things where dead people are kept, but “crypto-crystalline,” like crystals that are a mystery (as in “cryptic”). That’s because the crystals in this case are tiny little grains you can’t see—which makes them mysterious—and they’re packed together very closely. When you look at a cryptocrystalline mineral, you just see a smooth surface, not sharp-edged crystals. Minerals that have this kind of crystallization look like they’re massive. You can’t see a nice crystal shape, they might just be shaped like a plain old rock, and they’re often opaque.
quartz, sedimentary rocks like flint can be made of it too. Flint, a variety of the sedimentary rock chert, which is also mostly quartz, occurs in chalk and other soft sedimentary stone. It forms in gaps inside the chalk, as silicon (Si) dissolved in water collects and forms rounded lumps called “nodules.” Scientists think the silicon comes from the skeletons of ancient sponges–the living kind, as opposed to the ones in the supermarket–that built up along with all the calcium-based creatures that formed the chalk. When the chalk or other soft stone wears away, the hard flint nodules fall out. They can be white or kind of light gray on the outside, but inside they may be other colors, such as black, red, or brown, depending on what other minerals are mixed in. Flint takes advantage of the fact that when you chip quartz, it leaves a sharp edge. Indigenous people around the world where flint occurred realized this and chipped pieces of flint to create sharp points and blades for arrows, spears, knives, and axes. Later on in history, people also realized that if you hit a piece of flint in the right way against a piece of steel or pyrite, it would produce sparks. As a result, for a long time people have used it to start fires (such as campfires) and even to create the sparks that cause gunpowder to burn and fire a bullet from a gun.