*Please*. Do not use "unknown elements". That's Star Trek BS.I first read this quote a few years back and it greatly impacted my approach to alien technology in gaming.
In the Real World any element (or mixture thereof) is easily identifiable starting at around TL 5 or 6 (1910 technology).
Add in the fact that properties of elements are actually quite deterministic (that's how the periodic table was discovered in the first place!) and it gets sillier.
What works *far* better is to let them try and then tell them that even though the duplicate is an *exact* duplicate as far as they can tell, it doesn't work.
Why? Because there's something about it that their current technology can't even *detect*, much less duplicate.
An example would be someone from the 30s trying to duplicate a transistor, or worse yet an IC [integrated circuit]. As far as *their* technology can determine, the transistor is *absolutely pure* silicon or germanium (orgallium arsenide, or whatever). Their technology isn't up to detect the parts per *trillion* doping that makes it work.
With the IC, they can see that there's some sort of structure, if they use the best microscopes. No idea what it means, nor how to create such fine details. And without the theory, they can even determine *what*the structures should be.
Rubicon Models - New Preview
3 hours ago