I guess that depends on your definition of unique.
If you define unique as rare (and valuable), then gems such as alexandrite, grandidierite, jeremejevite, musgravite, and black opal are some of the most unique gems in the world, found in very specific, limited regions of the world. With these, finding them in a pure form is incredibly difficult, which is why they’re usually worth so much. Black opal is one of my favorites, since it looks like a rock swallowed a galaxy.
If you define unique more subjectively - on uniqueness of appearance rather than rarity - then there are a few other gems that stand out. To list just a few that are my personal favorites:
Ammolite: this gem is similar to opal – both fossilized shell-making, silica-rich creatures but in this case made from fossilized ammonites (nautilus) rather than diatoms (which are a kind of plankton) – but you can find it as whole nautiloids that can then be made into gemstone pieces (though I kind of prefer it whole but that’s my inner paleontologist speaking)
Fluorite: possibly my favorite gemstone mineral, fluorite is awesome for a couple of reasons - namely, it forms in near-perfect cubic crystals, is dazzlingly glowy under UV light, and comes in almost every color (usually clear, purple, and green, and occasionally a rare blue or impure yellow)
Hackmanite: this gem is a particular kind of sodalite that actually changes color in response to light (it’s called tenebresence, and it’s the same concept as transition lenses in glasses - just natural and in a really pretty gem) - when it’s mined, it’s usually purple, but over time, it turns white; if it’s put back in the dark, it’ll fade back to purple, and you can repeat this over and over and over
Labradorite: with this gem, light reflects off the crystal structure in such a way that the rock has really strong blue-green iridescence, and it looks different every way you turn it - the light will hit it in one spot and the dull grey rock will suddenly explode with all this amazing blue color
Pietersite: I love this gem because it can come in any color - from brown to yellow to red to blue to purple to grey - and I’ve never seen two pieces that look even close to identical. Technically, it’s a variation of quartz - a really, really awesome variation of tiger’s eye quartz that looks nothing like other quartz types
These are just a few examples, and this is a really long and rambling answer to a very short question, but I think uniqueness can be defined objectively in terms of rarity and worth, or subjectively in terms of how freaking cool and special they are
Ammonites at the Smithsonian. Originating from within the bactritoid nautiloids, the ammonoid cephalopods first appeared in the Devonian (circa 400 million years ago) and became extinct at the close of the Cretaceous (65.5 Ma) along with the dinosaurs. - Helen Rumeau - pinterest
Foot-long section of a large straight shelled nautiloid, both sides and end view showing the siphuncle. Based on the size of the section and the taper of the shell, this thing was probably 6 feet long or longer. I don’t know the species, but it’s from the Southern Lake Michigan/Illinois/Indiana region, likely Ordovician or Silurian in age. If you know what it is, I’d be happy to hear from you.
There is no glamor in eating garbage, no elegance in sifting through the sand for loose particles of decomposed weeds, or flesh rotting inside a nautiloid shell, or an osctracoderm’s curled droppings. Bumastus creeps on the seafloor with its own kind. They’re a near-sighted and nervous species. When the shadow of a eurypterid glides over them, the stout trilobites wriggle backwards into the sand, leaving just their eyes peeking from the speckled grains, watching anxiously for another shadow to sweep past. They’re cowards who would rather roll into a tight ball than fight when attacked. They distrust strange flavors, dislike different species. Neophobic, apprehensive, small-minded, and socially fussy, they just want to be left alone to pick through the sea’s waste and maintain their 60 million-year dynasty of bottom-dwelling.
Like their modern relative the chambered nautilus, Nautiloids had a soft body with tentacles that emerged from a hollow shell filled with gas for buoyancy. Unlike the Nautilus, which is admired for the beauty of its spiral shell, the Nautiloid shell came in a variety of shapes. Orthoceras (which translates as “straight horn”) Nautiloids had long, cone shaped shells, and must have swum like torpedoes through ancient seas. Their fossilized shells are common and are used as decorative stone all over the world. These distinctive black and white specimens come from Morocco, and are often quarried in slabs teeming with fossil Orthoceras shells. The originally hollow chambers of the shell have filled in with white calcite crystals, and in some pieces the siphuncle, a tube that ran through all the chambers of the shell, is visible.
Ilisine had thought itself clever, while the artificer Xalliordell worked on what he could with the decimated nautiloid Ilisine would look for more help. But as intelligent as it is Ilisine was not an adventurer without its ship at all.
Going alone and on foot on a planet it did not know much outside of books was a novelty to it. And even extensive prior planning of a route did not prepare it for the harsh reality of having to go alone on such travels with a lack of experience.
Its managed to avoid attentions of the curious by alternating between it was ‘against her religion to take off its face covering’ and ‘I am sick stay away’. And of course generally just keeping to the shadows….so thus Ilisine found itself completely lost in one of the cities on its route. It had taken an alley it did not know to avoid a crowd and now was standing there scratching its covered head.
Then it saw something that gave it pause, an abandoned building with red markings on it. Slowly shifting closer Ilisine wondered why the markings looked so much like Qualith.
Dear Lady helix. Are nautilus descendant of our dear lord helix and you? Thank you and Praise Lord Helix!
To answer your question directly, no, a nautilus
is not a descendant of an ammonite
(both of these are my beautiful bbys)
However, they are somewhat closely related. Within the class Cephalopoda, there are three sublcasses: the Coleoidea (squids, cuttlefish and octopus), the Nautiloidea (a diverse but mainly extinct group, with the exception of the modern day nautilus species) and the entirely extinct Ammonoidea, which of course includes the ammonites, and lord helix himself.
In fact, it is thought that both the Coeloidea and the Ammonoidea are descended from a particular group within the Nautiloidea, kind of opposite to what your question asked!
But like I said, though it’s not apparent today, the Nautiloids were a huge diverse group back in the palaeozoic day, and included not only those similar to the living reclict nautilus species, but also a variety of other forms, including the straight shelled orthocones (which in turn included this huge bastard)
Spiral shelled nautilids and ammonites superficially look very similar, however, there are key differences, for example, the siphuncle (a thread of flesh that runs through the animal’s shell, used growing the shell, and also for controlling water/gass input/output within the shell chambers i.e. controlling buoyancy) runs through the centre of a nautilid shell, as apposed to along the outer edge of an ammonite shell. There are also differences in the number of chambers within the shell, suture shape separating champers, and the fact that ammonites were thought to be able to completely retract within their shells - nautilids (at least living day ones) cannot.
anyway both groups are really super cool and I like cephalopods a lot, I hope that answered your question!