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The capital of a country is built on a series of stone hills that gradually lead to the ocean. The stone is noted for both the strength of castles made from it and its unique blue-green coloring. This city is surrounded by the ocean on one side, and steep mountains (which are responsible for the area's long rainy season) on the other.

What would be the best stone to use for these hills?

The first stone type is sedimentary. I think this one is unlikely, as sedimentary rocks don't usually exist in a blue-green color, aren't the best at withstanding erosion from constant rainfall, and generally aren't very good for building castles. Coquina, on the other hand, is one example of a sedimentary rock which would be good for building fortifications.

Next are igneous rocks. The color could come from underground mineral deposits, and the hills themselves could have been made from gradual lava flows. Igneus rocks can be very strong, withstanding both erosion and attack. Granite is a great example.

Metamorphic rocks are last. They can come in many different colors, with blue and green already being fairly common. While generally not as hard as igneus rocks, they could start as igneus rocks and morph over time, eventually being uplifted to the surface.

So, in summary, my question is this: What rock can I use that is strong, contains a mineral that makes it blue-green in color, and can form in a series of hills?

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    $\begingroup$ Why not granite as you said? There is one blue-green. $\endgroup$ – Alberto Yagos Jul 23 '18 at 6:40
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    $\begingroup$ @Willk Actually, any of the rock types can undergo metamorphosis. Even metamorphic rock can be further metamorphosed into a different type of subjected to the proper conditions $\endgroup$ – Dubukay Jul 23 '18 at 15:05
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    $\begingroup$ Forgive me, but why do you care what kind of stone it is? For the purposes of the story it's a hard fine-grained deep blue-green stone, and that's it. Is your audience composed mainly of geologists? How many people do you know who can tell the difference between basalt, granite and limestone? If you need a name for it just say it's a local sort of basalt and let it be; nobody would be able to say otherwise. ("Basalt" is just about any hard effusive rock.) $\endgroup$ – AlexP Jul 23 '18 at 16:22
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    $\begingroup$ All right, so in order to cover any objections just say it's basalt, the specific kind of basalt found in the Blue-Green Fine-Grained basalt belt which extends from the Upper End to the Lower End across the Realistic Land. "Basalt" is ambiguous enough and covers a wide enough spectrum of compositions that it could very well be the case that in the specific locality it has a distinctive blue-green lustruous color. $\endgroup$ – AlexP Jul 23 '18 at 16:29
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    $\begingroup$ @AlexP is 100% right. This type of basalt is often called "Greenstone" so... yeah. en.wikipedia.org/wiki/Greenstone_belt $\endgroup$ – kaine Jul 23 '18 at 16:51
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Can I recommend some serpentinite?

Literally named for its greenish color and certainly bluish under certain lighting, serpentinite is your best bet for a couple reasons.

First, it’s highly durable. Serpentine group rocks are metamorphic, forged at depths of up to 60km inside the earth and exposed by uplift and erosion. This process often leaves the serpentinite exposed on the surface, and its durability means that it persists even when the other nearby rocks have worn away, making them popular amongst rock climbers.

Turtle Rock, a serpentinite formation in Marin county

The tendency to produce individual, boulder-sized rocks gives another excellent reason to make castles out of it - you just have to pick them up and pile them in the right way. Their abundance and diversity means that mining and excavation is minimized without compromising on quality or sturdiness. You’ll still want a few carvers on hand, but the hard work has largely been done for you. Here’s an example of a serpentinite building: it’s held up for a few hundred years so your castle will have an acceptable lifespan as well.

Serpentinite building in Philadelphia

Additionally, serpentine minerals are already found in rolling hills and near oceans. They’re also famous for specialized and exotic plants that have grown accustomed to the high levels of cobalt and cadmium in the hills. This makes for some cool botany already even if it isn’t a part of your story.

If true serpentinite isn’t blue enough, well, it’s metamorphic! Mix some blueschist in there and you’ll get a darker, bluer stone. Blueschist is a bit more fragile than serpentinite because it has a few planes of good cleavage, but after some significant serpentinisation it’ll probably be nearly as hard. Fortunately, blueschist forms under similar conditions and locations as serpentinite.

blueschist

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    $\begingroup$ Most of the Serpentine Group is pretty soft not great as a building material, also includes asbestos, usually mixed into other group members like talc, this could be a very hazardous building material indeed. $\endgroup$ – Ash Jul 23 '18 at 12:42
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    $\begingroup$ @Ash I doubt castle builders would worry about shorting their life working with asbestos $\endgroup$ – Reed Jul 23 '18 at 13:09
  • $\begingroup$ @Reed The landholders and garrisons who live in the castles would be concerned that they, and their heirs, kept dying young and in horrific pain though. $\endgroup$ – Ash Jul 23 '18 at 13:12
  • $\begingroup$ @Ash From Wikipedia: “Serpentines find use in industry for a number of purposes, such as railway ballasts, building materials...” I dunno what to tell you except that some engineers think it’s a good idea. And if you’re already going to die at ~27 you’re probably going to have bigger problems than some asbestos. $\endgroup$ – Dubukay Jul 23 '18 at 14:59
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    $\begingroup$ Asbestos, left by itself in walls and structures, is not a health concern. With its excellent isolation capability, which it retains on even extreme heating - it is a spectacular fireproof / refractory material. Arguably, it is probably safer and keeps your home better than modern alternatives. The problem is mining it, transporting it, installing it, and decomissioning it. $\endgroup$ – Stian Yttervik Jul 23 '18 at 20:44
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Granite comes in a variety of colours, green among them. Unless your kingdom is sitting on a veritable bonanza of emerald and turquoise, I'd lean towards granite.

Green granite

From what I've read, granite doesn't come in blue. Other types of stone are marketed as "blue granite".

I think you could argue the stone above kind of looks glasy-greenish-bluish.

Granite has been long used in fortifications, of which the castle is a type:

Peter and Paul Fortress in St. Petersburg

Granite also has the benefits of being durable and of not being a source of asbestos, which is of concern in, for example, serpentine.

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  • $\begingroup$ The problem is that I would like it to be a little more blue. Sorry if this sounds nit-picky. I wonder if the blue not-really-granite has a similar hardness to real granite. $\endgroup$ – Starpilot Jul 23 '18 at 6:51
  • $\begingroup$ @Starpilot Like this: galleriaofstone.net/natural-stone/granite/azul-bahia or like this watertowntile.com/product/lemurian-blue? $\endgroup$ – Alberto Yagos Jul 23 '18 at 7:25
  • $\begingroup$ @AlbertoYagos More of the second one, but a little lighter. I guess the exact shade isn't important as it can fluctuate quite a bit. $\endgroup$ – Starpilot Jul 23 '18 at 7:29
  • $\begingroup$ @Starpilot Well, they are granite, I think it is a good solution for your material. Although maybe a bit heavy for tall walls in a castle. I don't know about construction, though. $\endgroup$ – Alberto Yagos Jul 23 '18 at 8:38
  • $\begingroup$ @Starpilot, You wouldn't use granite for a castle anyway, it's too hard to work with. Sandstone or Limestone are much more common, so the not-really-granite being a bit softer than real granite would actually make it better in practice. $\endgroup$ – Separatrix Jul 23 '18 at 9:10
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Have them build a log castle out of petrified wood.

petrified log

http://museum2.utep.edu/archive/fossils/DDpetrifiedwood.htm

You can have a forest's worth of these. Giant, ancient trunks larger than any trees that exist on earth today. Any color you want, and many colors if you want that - whatever minerals produce the crystallization will confer their colors to the logs. The logs are stone and will last forever. They can be used as is, for a sweet log cabin look.

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Blue quartzite, perhaps? It is a hard metamorphic rock. The colour is fairly subtle rather than bright blue in most of the pictures I can find on the internet. Pale blue quartzite

I can't say for sure if the really strikingly blue polished examples are as the stone appears in nature, or if it have been enhanced by dye for the interior decor trade. Polished blue quartzite Another polished blue quartzite

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May I recommend Jasper, jasper is usually agatised clay of some description, (clay that has been cemented together by silica deposited by geothermal fluid) it varies in colour from red to bright blue depending on the exact mineralogy. It is very hard wearing in nature and fully capable of forming mountains in quantity. Jasper is mechanically comparable to the iron cemented sandstone they're using to build Guédelon Castle being very hard but relatively easy to work.

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Tourmaline. (enclosed in some mother rock)

enter image description here

Tourmaline is extraordinary hard : on mohs scale it has a rating of 7–7.5.

(between quarz and topaz) (10 would be diamond already)

Tourmaline is much harder than marble (3-4) (and most likely also harder than serpentinite and petrified wood as well)

it can have many colours but it can be blue-green colored as well.

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    $\begingroup$ Scratch hardness isn't the right measure for deciding if something will make for a good building material. You're looking for something with a decent compression strength and fracture toughness. High scratch hardness often indicates a very brittle material, which is not something you want when building castles. $\endgroup$ – Mark Jul 23 '18 at 20:53
  • $\begingroup$ I think that some britty crystals enclosed some mother rock will also have a decent compression strength and fracture toughness (the mother rock prevents the whole thing from brittling, only small crystals can brittle at once when hit by some force) $\endgroup$ – KGM Jul 24 '18 at 7:24

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