You are an ambulatory inside-out coral colony with delusions of grandeur signaling to your peers over a glorified mycorrhizal network we grew together.
A beautiful analogy ... unfortunately just an analogy. One book I liked in theoretical biology was "Cats Paws and Catapults" by Steve Vogel (978-0393319903) about how biology and technology adopt different designs due to use of different materials.
Simply put, both birds and planes have wings but one if a flexible membrane that flaps and the other is a fixed structure with a jet engine.
So you posit that "had to be chemically changed by the organism as part of it's metabolism" as a requirement to be part of it?
Otherwise, what makes building a flying machine with a fixed structure and a jet engine fundamentally different than incorporating environmental iron into a shell? https://en.wikipedia.org/wiki/Scaly-foot_gastropod
Good point, although it makes me wonder what a 'mechanical metabolism' might be.
I've heard of the worm with a fool's gold shoe before (or iron sulfide of some kind). There are also Antarctic fish that make antifreeze proteins that bind to ice crystals to stop them grow. Also trees make themselves by extracting smoke from the air using light and water.
Ok, so maybe I have to work on the analogies. However, my point is that organic systems (biological organisms) are embedded in physical systems. They can manipulate the environment to collect energy and use bits of it to repair themselves.
Talking of iron-sulfide, there are tiny cubes of Fe-S in many enzymes (https://en.wikipedia.org/wiki/Iron%E2%80%93sulfur_protein). I remember talking with my supervisor at Glasgow about the possibility of these being fragments of ancient catalytic machinery from when biology was much closer to the metal (as it were) and these parts have gradually been enveloped in protein.
I liked Alexander Cairns-Smith's book that describes a 'rope' of systems, starting with a thread of replicating clay that templates replicating RNA that leads to DNA and so on. It makes sense to me that the origin of life would have to involve the rocks and minerals. Nick Lane has some theory about serpentinite as well.
Summing up, biological systems can absolutely have metal parts. They even have rotational engines (of a sort) in flagella. However, biological and non-biological designs are often very different because of the different scales and levels of organisation. A human is organised across at least 7 levels (atoms, molecules, organelles, cells, tissues, organs, whole) where a tractor is really just (atoms, parts, whole). A fruit fly swims through the air due to its tiny scale, while a plan e forces its way through.
Apologies, bit of a ramble - I'm a little tired today.
> designs are often very different because of the different scales and levels of organisation.
This is our disconnect. I'm arguing for definitions that are scale invariant. In the long run and the wide (universal) scale I would expect any developing system to just increase in "layers of scale" over time so basing our labels on them doesn't seem useful.
In that view "technology" is just another natural specialist organelle produced by the biosphere.
As a fun aside, In the context of viewing the entire biosphere as an organism implies we are it's (still maturing) reproductive organs and that space colonization and terriforming will just be the organisms calfing season.
