One of the possible issues is that the chocolate contracts when cooling down, possibly changing or damaging the diffraction imprint.
Chocolate making is a whole art (and technique) onto itself. Producing fine chocolate requires advanced machines (at least for 19th/20th century standards).
For the whole process, pretty much all you need is:
1. an oven to roast cocoa beans (though you probably want a tabletop coffee roaster for reliable results)
2. a rolling pin to break apart roasted beans into nibs and husk (this does get old pretty fast after 5 batches of chocolate, so people recommend moving onto another kitchen countertop device, the Champion Juicer)
3. a hair dryer to separate husk from nibs
4. a melanger, as mentioned above, to refine the nibs (with any desired additives like sugar and milk powder) for 24-72 hours
5. a couple of bowls and a good thermometer for tempering
6. (optional) plastic molds for shaping your chocolate.
https://chocolatealchemy.com/how-to-make-chocolate-the-compl...
(Off-topic: is there a way to format a bulleted/numbered list to not require separate paragraphs?)
I was mostly referring to the presses that separate the cocoa butter from the rest. It's not an easy process (example https://www.youtube.com/watch?v=HzcN3cbikzA ). Though as I pointed out, it's probably 19th/20th century technology.
Most small batch chocolate makers don't bother with pressing their own cocoa butter because of the scale required to make it worth it. There's only so much cocoa butter you can press from beans, and then you need to figure out what to do with the remaining cocoa solids. (As an exception, Goodnow Farms is one that did invest in one for, I think, $50000; they sell the cocoa solids to restaurant chefs.) As a result, most of this is still in the industrial realm, even if the tech itself isn't particularly new (it hasn't changed much beyond what's in the linked video).