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Background: The planet is very volcanic, having at least a dozen volcanoes on any landmass and even more in the oceans. I want the planet to have very few metal ores but a complete overabundance of gemstones.

  • Just to clarify the planet does have all types of metals present, just in smaller amounts. There is a lack of light metals and a very large lack of heavy metals.

Question: Is it possible to have a planet with one but without the other? Or will it always have an equal mix of metals and gems?

If it’s not possible to have one without the other is there any reasonable way to keep my species away from the metal ores on the planet?

(Please tell me if the question seems too broad or could be improved. Thank you in advance.)

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  • $\begingroup$ Are there any requirements on what types of gems the planet should be abundant in and/or what types of ores the planet should be deficient in? $\endgroup$ – Shalvenay Oct 23 '17 at 0:09
  • $\begingroup$ @Shalvenay When it comes to metal ores preferably all of them, however, I doubt that's 100% possible so as many as possible should have deficiencies. It's mostly the same for the gems, an overabundance in as many as possible (however an increase -however small- in diamond is a must.) Other than that the 'what' and 'amount' of both is very flexible. $\endgroup$ – Axolotl Oct 23 '17 at 0:16
  • $\begingroup$ Also, do you want most of the gemstone minerals to be in gem-grade form, or in their lower-grade industrial forms? $\endgroup$ – Shalvenay Oct 23 '17 at 0:32
  • $\begingroup$ @Shalvenay Industrial most likely (it means they're full of flaws and not good for jewellery but good for practical things, if I'm not mistaken?) They make use of the gemstones in normal life since they're more easily found and are cheaper than metals. (They don't fully replace metals, however.) $\endgroup$ – Axolotl Oct 23 '17 at 0:58
  • $\begingroup$ Does the planet need to support life? Removing the metals makes life unlikely. $\endgroup$ – John Oct 23 '17 at 3:53
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It's hard to have a lot of volcanism without having heavy metals, the heat that drives volcanism comes from the decay of radioactive materials in the core, these materials decay to form stable heavy metals. If you go with a cosmological approach rather than assuming chance impoverishment then in order to avoid abundant metal in a planet you have to look to an early generation star (a red dwarf like Barnard's Star for example) which has very little metal and then sink the metal you have into a strongly magnetic core that supplies a high magnetic field strength to protect from solar flares etc... Such a world will have Diamonds in the crust like they're going out of fashion due to the relative abundance of Carbon but little to no active tectonics or volcanism.

Now to the core question, most of the gems we have these days are Silicates, Rubies and Sapphires are an exception being impure Aluminium Oxide and Diamonds are pure Carbon, but Zircons, Emeralds, Opals, Topaz, Amethyst etc... are all Silicate minerals, a major increase in the amount of Silicate mineralisation could tie up large quantities of Iron, Aluminium, Chromium, etc... leading to gem formation in place of metal ore deposits in the form of oxides, sulfates, and sulfides. This is one of the methods suggested in this question for removing accessible metal ores from an otherwise active world.

For potential volcanism on a world without heavy metals see Carbonatite volcanic chemistry, they aren't exactly the volcanoes one is used to thinking about but they're dangerous enough.

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  • $\begingroup$ If you really want volcanoes on your gemworld, you could make it a (large) moon of a gas giant, with an orbit that gives it significant tidal heating. For instance Europa has a crust composed primarily of silicates, and internal heat from tidal flexing. (It's also small and frozen over, so you might want to fiddle with your star a little...) $\endgroup$ – Cadence May 27 '18 at 7:15
  • $\begingroup$ @Cadence Um Europa is, inference to best guess, salty water covered in reasonably solid ice, but yes there does appear to be some heating, presumably from tidal flexing taking place. Io has a silicate crust, and liquid sulfur volcanism almost certainly heated by tidal flexing. The thing is both of them are tiny compared to the gravity field they're in that causes them to flex, and under the influence of repeated orbital resonance events. I'm not sure if something Earthlike in size and having a differentiated crust, mantle, core, structure would respond the same way, it might just pull apart. $\endgroup$ – Ash May 27 '18 at 13:48
  • $\begingroup$ The Roche limit - the minimum orbital distance where tidal forces would rip a satellite apart - actually gets closer the more the satellite masses compared to the primary. That is, a heavier moon could orbit closer without any problems. Whether it would have notable tidal heating is another question of course. (It doesn't seem to depend on mass from what I can tell, only orbit.) Europa also seems to have a metallic core and thus, at least some differentiation. That's not to say this scenario is accurate, but it's plausible enough for mid-grade scifi. $\endgroup$ – Cadence May 27 '18 at 14:13
  • $\begingroup$ @Cadence Huh I had that messed up, I thought it was satellite density not mass, so yeah you could get right up close to a gas giant and get another heavy satellite that stretches it the other way during resonance events and get quite a bit of internal heating. Being right close to a hot gas giant (like Jupiter only more so) would also help with the planetary energy budget, you could be farther out from a given star that way. $\endgroup$ – Ash May 27 '18 at 18:55
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Refining bauxite into aluminum is an extremely involved process, and corundum is aluminum oxide.

So, if your planet was rich in aluminum but poor in iron and other heavy metals (at least in the crust, you definitely want iron in the core), then the inhabitants might find lots of rubies and sapphires lying around, but without any "easy" ores to refine into tin, copper, or iron, they might never develop metallurgy needed to advance out of the Neolithic.

A big problem is that the red color of rubies comes from chromium, and the blue of sapphires comes from iron and titanium.

Another problem is that the main igneous source of corundum is nepheline syenite which is relatively rare since it comes from a silica-poor magma. Magma containing a lot of silica gobbles up the aluminum and produces feldspar instead of nepheline. It's very, very difficult to imagine an Earth-like planet with a silica-poor crust! Of course, an incrementally larger amount of aluminum might produce enough more nepheline for your purposes.

Not only that, on Earth, crustal iron played a big role in the evolution of Earth's atmosphere. As cyanobacteria produced oxygen in the Archaean and Proterozoic, the oxygen combined with iron dissolved in the water and settled to the bottom, producing the banded iron formations that have been a major source of iron ore throughout history. Without the iron, the atmosphere might have too much oxygen!

Finally, vertebrates on Earth need iron in hemoglobin molecules in carry oxygen to their tissues. Other creatures use other pigments in their blood, based on copper. I don't know if there would be a suitable aluminum or magnesium based pigment.

Finally, the first iron worked by humans was "skymetal"; that is, it came from meteorites. It's difficult to imagine a planetary system containing an Earth-like planet without iron meteorites. But they were rare and iron objects made from them might be just enough iron for your world.

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It would be unrealistic to assume that a planet would lack all metals. If it did it would be a strange world indeed because most of the periodic table is composed of metals. In addition rubies contain aluminium and emeralds contain both aluminium and beryllium.

Assuming you mean just a lack of heavy metals such as iron, copper, cobalt and gold etc, I think it would still be unreasonable for them to be absent. However they might be present in much smaller quantities.

If the crust were thicker and the core smaller or even missing entirely then the lighter metals might predominate and the heavy metals might be much rarer.

In a volcanic world it would certainly be possible to find a lot of diamonds as they are often found in the so called diamond pipes – which originate volcanically and diamonds are only composed of carbon (impurities aside).

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