iMac G3 (Bondi Blue): What Kind of Plastic is the Shell?
I believe that the plastic casing is polyvinyl chloride (PVC) because of it's durability and physical characteristics.  However, I don't have access to that machine to do any definitive testing.  That said, with a small sample of the plastic, you can determine what type it is with some very simple tests you can do at home;  see below.

(I don't believe it to be Lexan (acrylic plexiglass) due to how easily it scratches with a fingernail.)



Personal note:  Way back when, I worked for an electronics manufacturer that reverse engineered products to see how they could make them cheaper (they were based in the good ol' USA).  One of the engineers taught me how they identified certain materials like plastics.  I had to go look this up since it was so long ago I actually used this type of info.

Basic Plastic Testing1

There is some basic tests you can do to plastics to identify the type of plastic material.  Basically, the test involves:


Look:  Is it transparent, translucent or opaque?
Feel: Does it bend? Can it be scratched? What does the surface feel like?
Cut:  Does it cut easily? Are the edges smooth or jagged? Does it crumble or flake?
Float:  Does it float or sink? (Note: not applicable to expanded foam materials. Water should be around room temperature).
Burn:  What is the size and colour of the flame? Do molten drips fall from the sample and continue to burn? Does the sample self-extinguish? Is there any odour when the flame has been extinguished?


Low density polyethylene (LDPE)   


Transparent only as thin film, translucent in thicker sections
Fairly flexible; soft, 'waxy' feel, easily scratched
Easily and smoothly cut
Floats
Not self-extinguishing; molten droplets which usually go out on reaching bench or floor; blue flame with yellow tip and little smoke, smell of burning candle/paraffin when flame is extinguished


High density polyethylene (HDPE)  


Transparent only as thin film, translucent in thicker sections
Fairly stiff and hard, can be scratched by fingernail
Easily cut with smooth edges
Floats
Not self-extinguishing; molten droplets which usually go out on reaching bench or floor; blue flame with yellow tip and little smoke, smell of burning candle/paraffin when the flame is extinguished


Polypropylene (PP)    


Transparent only as thin film, translucent in thicker sections
Stiff; hard, can be scratched by fingernail
Easily cut, fairly smooth edges, when cut with chisel leaves white mark
Floats
Not self-extinguishing; molten droplets which usually go out on reaching bench or floor; flame mainly yellow with a trace of clear blue at the bottom; smell of burning candle/diesel when flame is extinguished.


Polyvinyl Chloride, Unplasticised (uPVC)


Transparent (unless fillers or pigments have been added)
Stiff; hard
Fairly easy to cut, smooth edges
Sinks
Burns with difficulty, self-extinguishing; yellow flame, blue-green at bottom edges; unpleasant, acrid odour of hydrochloric acid.




1 How, using simple manual tests, can I identify an unknown plastic material?