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Have a creature from a dream. It's a carbon based humanoid creature partly made of the mineral olivine. Could there be any reason it might contain this mineral? Perhaps a show of health - the more olivine coloration in its skin the healthier it is since it can find more olivine-containing food?

I remember this creature has blue blood; I believe that could mean it uses copper in its blood and uses liquid ammonia instead of water. Since the creature's planet has large amounts of liquid ammonia, the planet has more pressure than Earth's. (Information here in case it affects the answer)

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    $\begingroup$ A creature made of olivine is not credable without the use of magic. $\endgroup$ – Slarty Nov 18 '19 at 18:07
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    $\begingroup$ Human skin weathers and alters into other things in the presence of water very much faster than olivine, and yet we swim just fine. When a mineral is said to weather "readily" in the presence of water it does not mean that it dissolves like sugar; for example, iron weathers readily in the presence of water. (Olivine does dissolve quickly in hot water saturated with carbon dioxide, but then human skin also doesn't fare well in such conditions.) $\endgroup$ – AlexP Nov 18 '19 at 19:13
  • $\begingroup$ your big problem is olivine is unstable at surface pressures in water, so forming it in an organism is all but impossible. $\endgroup$ – John Nov 19 '19 at 6:14
  • $\begingroup$ @John if this creature's planet was under more pressure would it work? $\endgroup$ – Chickenpeep Chickenpeep Nov 19 '19 at 23:20
  • $\begingroup$ @ChickenpeepChickenpeep not if you want liquid water. $\endgroup$ – John Nov 20 '19 at 3:09
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Some sea creatures use defensive strategies involving hard objects, such as shells, cans, bottles, and whichever they can find and suits their size, doesn't compromise mobility and are easily available.

Your creatures can be an evolved version of this. They used stones as defense, a myriad of them on their skins, as scales in a reptile. As intelligence, and fashion came to arise, they preferred gemstones, in particular hard olivines, with maybe a touch of other gemstones to indicate status, wealth, profession, or whatever your storytelling needs.

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A simple idea could be some kind of dense mucus or slime, produced by the creature's glands in its "skin" pores - I'm thinking similar to the mucus in human stomach lining, just on the outside... Might be oversimplifying it but in certain habitats it could work (especially if the use of tools is not too extensive; what do you mean by "work"?)

About the buoyancy:

I can state flatly that heavier than water swimming creatures are impossible.

In other words, good muscle and good hydro-dynamics (proper shape) will do the trick!

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    $\begingroup$ Heavier-than-water things swimming is no more impossible than heavier-than-air things flying. What's that quote from? I don't see it in the question or comments. $\endgroup$ – Starfish Prime Nov 18 '19 at 18:19
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    $\begingroup$ Exactly, I just misquoted Lord Kelvin ;) $\endgroup$ – BIOStheZerg Nov 18 '19 at 18:21
  • $\begingroup$ Ahh, I understand now ;-) my bad. $\endgroup$ – Starfish Prime Nov 18 '19 at 18:21
  • $\begingroup$ Muscles made from Olivine will not be mobile $\endgroup$ – Slarty Nov 18 '19 at 18:24
  • $\begingroup$ True that, I was thinking more about the lines of an exoskeleton or carapace... $\endgroup$ – BIOStheZerg Nov 18 '19 at 18:27
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As long as the creature weighs less than the liquid it displaces, it will float. You can take a material of arbitrarily high density and craft something that will float, so long as you make its volume large enough. Metal ships are made of material that's much denser than water, but they're designed to have a large cavity inside so that they displace a lot of water, making it so that the ship weighs less than the water it displaces.

Perhaps your creature has air or vacuum pockets inside its body that lower its net density. Depending on the body plan, you could make a creature made of any material float in any liquid you like.

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