Growing Spectacular Gem-Like Crystals From Rust And Simple Ingredients

Potassium ferrioxalate crystal

When we talk about crystals around here, we’re generally talking about the quartz variety used to make oscillators more stable, or perhaps ruby crystals used to make a laser. We hardly ever talk about homegrown crystals, though, and that’s a shame once you see how easy it is to make beautiful crystals from scratch.

We’ve got to say that we’re impressed by the size and aesthetics of the potassium ferrioxalate crystals [Chase Lean] makes with this recipe, and Zelda fans will no doubt appreciate their resemblance to green rupees. The process starts with rust, or ferric oxide, which can either be purchased or made. [Chase] chose to make his rust by soaking steel wool in a solution of saltwater and peroxide and heating the resulting sludge. A small amount of ferric oxide is added to a solution of oxalic acid, a commonly used cleaning and bleaching agent. Once the rust is dissolved, potassium carbonate is slowly added to the solution, turning it a bright green.

The rest of the process happens more or less naturally, as crystals begin to form in the saturated solution. And boy, did they grow — long, prismatic lime-green crystals, with a beautiful clarity and crisp edges and facets. The crystals don’t last long under light, though — they quickly lose their clarity and become a more opaque green.

[Chase]’s crystal-growing efforts have shown up here before, when he turned humble table salt into beautiful cubic crystals. We find the whole crystal-growing process fascinating, and we’re looking forward to more of this in the future.

33 thoughts on “Growing Spectacular Gem-Like Crystals From Rust And Simple Ingredients

  1. Copper(II) sulfate pentahydrate is readily available as root-killer from the local farmer’s co-op. Dissolve in water, hang a tiny seed crystal from a nylon monofilament or similar, allow solution to evaporate very slowly. Beautiful deep-sapphire-blue crystals.

      1. You let some of the solution dry out on a piece of paper and it’ll form tiny crystals. You use tweezers to pick the nicest-looking one.

        I did this at school, using various chemicals borrowed from the chemistry lab (with the teachers’ knowledge; although even back then, they probably shouldn’t have been giving it to me as most of them, including copper sulphate, is toxic) and grew them in a jam jar in my room. They do grow very slowly, or at least they do in a Scottish boarding school where the air temperature is barely above the sublimation point of CO₂.


        This describes the basic process. Use room-temperature distilled water, not hot tap water, to make the saturated solution. Leave a little solid copper sulfate at the bottom of the jar to ensure that the seed crystal won’t dissolve.

        For a perfect crystal dry some of the solution as David Given suggests above. Pick a tiny crystal that appears to be as perfect as possible, and superglue it to the end of a nylon monofilament. Hang it in the solution and cover the jar with cloth, so that evaporation is slowed and to protect from dust (which will act as seed crystals, albeit irregular ones :-(). The best crystals will be formed when the crystal can grow very slowly.

        After the crystal has grown for a while you can see whether it will be a good one. You will probably have to repeat the process several times to get one that looks perfect.

        FWIW the solubility of copper(II) sulfate pentahydrate at 20 C is about 315 grams per liter of water; that’s about how much is needed to make a saturated solution. I would suggest setting up several pint jars for crystal growing. And the copper sulfate solution as well as imperfect crystals can be reused as long as it’s not contaminated.

      1. You’ve got to be kidding me, right? Sure copper sulfate can be toxic, but I’m not going to be putting it in my mouth and sucking on it. I’m only picking it up and will rinse my hand later. Copper is actually needed in the human body in trace amounts, so accidental poisoning is pretty unlikely unless you’re eating it. And I have actually used copper sulfate in our swimming pool and hot tub (1 or 2 ppm) to cut down on chlorine and bromine use – with no ill effects of course.

        1. Also copper kills bacteria , they used it when soldiers got wounded in the war. The would grind it up and pack the wounds with it. Also kills the covid 19 virus . Hospitals are using it a a lot to kill germs on handles and sinks 🤷can’t be that toxic

  2. Those crystals are beautiful.

    Several thoughts:

    Is this material piezoelectric? How hard is it–I’m guessing it could be cut with a Dremel wheel and ground to thickness with emery paper. I wonder if you could make an oscillator crystal out of this or, create a bimorph (as is done with rochelle crystals) to drive a set of headphones.

    What would happen if you cast one of these crystals into a block of styrene casting resin. Would it preserve it? Would the crystal off-gas on decomposition and fracture the casting?

    What would happen if you filled the bottom half of the “growth” jar with those water-absorbing silica gel beads (the kind used by gardeners to retain water in flower pots)–what I’m proposing is soaking the beads in the growth medium, filling the jar half-full with the beads, then topping off the jar with more medium. A seed crystal could be dropped in, and would lay on the surface of the submerged beads.

    Would the gel provide a gentle enough “bed” for crystal growth that would allow crystal formation on all sides without the use of a string or wire to suspend it?

    1. If I remember 70s high school chemistry correctly, Rochelle Salt was pretty easy to grow in large crystals. Ammonium dihydrogen phosphate was another one and can be used as a laser frequency doubler under the right conditions.

  3. Potassium ferrioxalate is (or at least was) used as an actinometric reference material in chemistry – it decomposes in a predictable way when illuminated by uv light, so you can make up a solution of known concentration, shine light on it, then measure the concentration again (by titration) and back calculate how many photons ended up getting captured.

  4. I wonder if crystals could be suspended by inertia by strategically turning a closed container. This way the crystals have a truly isotropic environment and would grow into cubes not slabs.

    For example, make a cubical container from acrylic sheet and o-rings for a seal on one or two of the faces. Then make a motorized setup that can turn the cube in any direction, something like the automated Rubik’s-cube solvers. Finally, take pictures with a webcam, detect the crystal, and time the turning motion to keep the crystal always centered in the container. Depending on the viscosity, the motor system might have to be pretty agile.

    Of course some way must be found to remove the solvent, otherwise nothing will happen. Maybe some capillaries could be arranged that would allow solvent to slowly evaporate and air to enter. The bubbles could perhaps be kept close to the container’s boundary so they wouldn’t interfere with the crystal in the center.

    1. Crystals generally grow by crystal packing theory. It’s about how to fill a box with the most stuff and least space. And most of these crystals are going to be d-orbital metals so it will depend on the angle of those bonds.

    1. If the only degradation is caused by UV light, you may be able to grow them in a dark room environment and apply a clear epoxy or polyurethane that has fillers with UV shielding properties.

  5. The book “Crystals and Crystal Growing” by Alen Holden and Phylis Singer is probably the best guide on the subject. Wonderful book, very detailed. I highly recommend it. It’s available used on Amazon very inexpensively.

  6. One of my favorites is aluminium potassium sulfate crystals. You can buy it as a bag of powder or small crystals, but it’s the primary ingredient of solid crystal underarm deodorant.

  7. Rochelle salt is also interesting because it’s piezoelectric. You can do the chemistry at home, bicarbonate of soda is baked and then you add cream of tartar. There are several recipes online. I’ve not made any for over ten years. Tempting to give it a go at some point, but it’s got to bake for quite a while.

    1. This crystal would not be suitable as a gain medium for a laser, in fact it is degraded permanently by exposure to light.

      I think the easiest DIY laser to make is a TEA nitrogen laser (emits in the UV), but the build is a little bit dangerous if you are not familiar with high voltage. Output power and beam quality is also not great.

      Another easy laser to build is a ruby laser (emits in the red), but you would need to buy the gain medium. It’s not really possible to grow a high-purity ruby single crystal in your garage. But you can buy ruby rods designed for lasers for relatively cheap on eBay. Then all you need is the right mirrors and adjustable mounts, and a way to pump the gain medium (usually a flash lamp).

      Dye lasers (where the gain medium is a solution of organic dye in a solvent, instead of a crystal) are also fun to make but very messy. It’s basically impossible to work with a dye laser without getting stains on everything, and the dyes used are often toxic. What’s nice about dye lasers is that they are tunable because the gain bandwidth is pretty large, and you can experiment with different dyes to get various laser colours.

      For building your first laser I think ruby is the way to go. Check out Sam’s Laser FAQ for information on laser theory and practical design tips.

  8. You guys are all way too young. I used to have crystal-growing kits my dad bought at the five and dime. They grew up from the dish. It was fun. I also used to hang a string from a skewer down into a supersaturated sugar solution, with or without flavoring and coloring. The resulting crystals are rock candy, and who cares if it’s perfect, it’s delicious! It’s always preferable to be able to enjoy your failed experiment for dessert, if possible.

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