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I'm working on a setting in which exists this cabbal of forgers that can work metal with the power of their mind in secret rituals that no man outside their circle can even imagine. They are known for creating strong and marvellous metals, like bronze and steel, and other much more misterious and expensive, like the selenite, a strong metal of a beautiful pale blue color.

Now, these guys embelish and create these legends about their work so they can appear as powerful mages able to create super-metals, but in reality they are just good at creating high quality alloys and add some nice color to them so it can look more miche and unique. My question is, what metals should they use to obtain a metal like the selenite? Such metal should be:

  • As strong, tough and flexible as high-quality steel (even more so, although I believe that's not really feasable).
  • It needs to have a pale blue color, and it can't be added as a coating.
  • It needs to be fit for sword crafting.
  • In a perfect world this metal would be also pretty light, but I guess I'm starting to ask for too much, so I'm willing to sacrifice this point and even accept it being slightly heavier than steel.

Maybe an alloy of iron, carbon and some cobalt could do the trick? Is it even possible to have such metal?

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    $\begingroup$ (1) Cobalt is not blue. It is silver colored, like most metals. No metal is blue; copper is red, gold is yellow, everything else is silver-ish. (But most metals tarnish quickly, losing their silver-ish lustre.) (2) There are already a mineral called selenite (kind of gypsum, hydrated calcium sulfate), and an entire class of compounds called selenites, notably silver selenite and sodium selenite. $\endgroup$
    – AlexP
    Commented Oct 25, 2022 at 14:17
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    $\begingroup$ ... But steel can be blued, and this is a common passivation process. The blue layer is not "added", it is created in place by a suitable treatment. $\endgroup$
    – AlexP
    Commented Oct 25, 2022 at 14:22
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    $\begingroup$ FYI: cobalt ruins steel and releases poisonous gasses when smelted. This metal gets its name from the kobolds of German folklore who were believed to be mischievous spirits. Cobalt contamination was very difficult to detect or separate from iron using medieval techniques, so when a batch of steel was mysteriously ruined by it, it was blamed on these spirits. $\endgroup$
    – Nosajimiki
    Commented Oct 25, 2022 at 22:00
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    $\begingroup$ Blue looking things are usually a refractive effect. Blue minerals contain metals, usually, but aren't alloys, by definition. Can't you just make one up for your story? $\endgroup$
    – user458
    Commented Oct 26, 2022 at 16:00
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    $\begingroup$ Does it 100% have to be metal? There are blue materials that might be suitable for making a "sword" in a different sense - e.g. the Aztecs made swords which consisted of a wooden core with shards of obsidian inserted into grooves around the edge. These shards could break and fall out in use, but were easily replaced. Obsidian is too brittle to make the whole blade, but it's one of the sharpest materials known to man, and there are varieties of it that are naturally blue. $\endgroup$
    – Darrel Hoffman
    Commented Oct 26, 2022 at 19:15

4 Answers 4

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Blue steel is a real thing

Blue Steel is a steel alloy containing Iron, Carbon, Tungsten, and Chromium sometimes used in specialty knife and sword manufacturing. When properly tempered, it has a superior toughness, spring, and edge retention to carbon steels of similar hardness. T10 steel is an example that's well known for making premium sword blades.

The blueness of blue steel is only slight in most cases. The causal observer may not notice the color difference at a glance, but it is pretty apparent when you see the two alloys next to each other. That said, just like a jeweler knows right away if he's looking at a diamond or a cubic zirconium, sword experts in your setting will immediately recognize the faint blue tint of a blue steel blade without needing a reference point.

enter image description here.

Side Notes:

  • Blue steel is distinctly different from Blued steel. Blued steel is a surface process to color the steel blue. Blue steel is an actual bluish alloy.
  • Blue steel may not be an ideal steel for every type of sword. Every example of blue steel that I can find has a relatively high hardness. This may make it too hard to be a good choice for something long and skinny like a longsword or rapier, and would likely chip on a hard impact. Such swords are best made out of softer medium carbon steels. But for a shorter, broader, or more curved style of sword like a Katana, Gladius, Cutlass, Scimitar, etc. it should perform extraordinarily well.
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    $\begingroup$ This is most interesting! Do you have a reference? What is this alloy called commercially? $\endgroup$
    – AlexP
    Commented Oct 25, 2022 at 18:17
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    $\begingroup$ @AlexP Blue steel is a blanket term meaning any steel alloy containing Tungsten and Chromium. Many commercial tool steels fall into this category. In America, steels prefaced with "W" are tungsten steel alloys; so, those like W1 which also contain chromium would be blue steels (some are less blue than others). T10 is a Chinese tool steel that has proven results in sword making which is why I used it as the exact example in my answer. There's also Aogami Super which is a darker blue steel popular in Japanese knife making, but I believe it's too hard and brittle to be good for swords. $\endgroup$
    – Nosajimiki
    Commented Oct 25, 2022 at 19:09
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    $\begingroup$ Thank you, I have learned something. $\endgroup$
    – AlexP
    Commented Oct 25, 2022 at 19:51
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Cobalt won't do it: metallic cobalt is silver-grey and cobalt blue is CoAl2O4, which is non-metallic. There aren't any naturally blue metals or alloys that I can find which are remotely suitable for swords. "Blue gold" can be made as an alloy of gold with either gallium or indium. It will be far too heavy and soft for swords.

You seem to be stuck with surface treatments or coatings. Ordinary blued steel is far too ordinary. If your secret craftsmen have access to titanium and electricity, then it's possible to produce some very nice blues with anodised titanium.

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    $\begingroup$ Titanium is not as good of a sword making material as steel is. While it is tougher for its weight than steel, you still need to make it thicker than steel to get the same blade strength which is bad for cutting. This "less dense" toughness also leads to poorer edge retention. $\endgroup$
    – Nosajimiki
    Commented Oct 25, 2022 at 17:34
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    $\begingroup$ Titanium cracks more easily than bends, it's too inelastic. Maybe a good spear, but not a sword. $\endgroup$
    – user458
    Commented Oct 26, 2022 at 16:03
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Case hardening hardens the surface of steel and leaves the inner parts softer, and more importantly discolours the surface of the metal. In real world the the result is rainbow colours in unpredictable patterns. The cabal's secret methods give them perfect control of the temperature and chemistry of the process, which makes their swords both stronger and gives them one solid colour.

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    $\begingroup$ The color resulting from case hardening is only random under uncontrolled conditions. But, it can be controlled by the temperature and environment you expose the metal to during tempering and by the quenching method. This is one of the methods of making blued steel referenced in other answers, but only creates a blue coating, not a solid blue metal. $\endgroup$
    – Nosajimiki
    Commented Oct 26, 2022 at 13:58
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    $\begingroup$ Yes. I interpreted the setting where secret cabals exist and mysterious materials like selenite are taken seriously as a low tech setting where controlled hardening could count as the kind of flashy but not really that superior process that the OP is asking for. Also, I thought it is worth separate answer because while the other bluing methods are for corrosion protection, it actually related to hardening. $\endgroup$
    – ojs
    Commented Oct 26, 2022 at 14:17
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https://en.wikipedia.org/wiki/Tempering_(metallurgy) enter image description here

Depending on the temperature steel was heated it gets a different coloration.

If steel has been freshly ground, sanded, or polished, it will form an oxide layer on its surface when heated. As the temperature of the steel is increased, the thickness of the iron oxide will also increase. Although iron oxide is not normally transparent, such thin layers do allow light to pass through, reflecting off both the upper and lower surfaces of the layer. This causes a phenomenon called thin-film interference, which produces colors on the surface. As the thickness of this layer increases with temperature, it causes the colors to change from a very light yellow, to brown, then purple, then blue. These colors appear at very precise temperatures, and provide the blacksmith with a very accurate gauge for measuring the temperature. The various colors, their corresponding temperatures, and some of their uses are:

Faint-yellow – 176 °C (349 °F) – gravers, razors, scrapers
Light-straw – 205 °C (401 °F) – rock drills, reamers, metal-cutting saws
Dark-straw – 226 °C (439 °F) – scribers, planer blades
Brown – 260 °C (500 °F) – taps, dies, drill bits, hammers, cold chisels
Purple – 282 °C (540 °F) – surgical tools, punches, stone carving tools
Dark blue – 310 °C (590 °F) – screwdrivers, wrenches
Light blue – 337 °C (639 °F) – springs, wood-cutting saws
Grey-blue – 371 °C (700 °F) and higher – structural steel

Beyond the grey-blue color, the iron oxide loses its transparency, and the temperature can no longer be judged in this way. The layer will also increase in thickness as time passes, which is another reason overheating and immediate cooling is used. Steel in a tempering oven, held at 205 °C (401 °F) for a long time, will begin to turn brown, purple or blue, even though the temperature did not exceed that needed to produce a light-straw color. Oxidizing or carburizing heat sources may also affect the final result. The iron oxide layer, unlike rust, also protects the steel from corrosion through passivation.

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