Celluloid is one of my favorite topics since I'm a fountain pen collector.
There are two types of celluloid, celluloid nitrate and celluloid acetate. Celluloid nitrate is the stuff with all of the cachet; it is also the version that is highly flammable. Celluloid acetate looks almost as good but does not have the cachet. A lot of modern celluloid is celluloid acetate. Camphor is used as the plasticizer in celluloid nitrate hence the Vick's smell. Celluloid is made in a different way than modern plastics. Celluloid production is more akin to making pasta since you start by making a paste that color is added later. The rolled sheets can be rolled and cut and combined with other sheets of different colors. This process enables celluloid to be made in patterns that modern plastics cannot readily replicate. Many of the great celluloid plastics used in vintage fountain pens can never be made again. That's why I often pay $2k for a vintage pen in celluloid.
Mazzuchelli is the Italian company mentioned above. They can still make the celluloid but the price is high. You have to buy a huge quantity and the stuff is expensive to make and hard to make on a scale like modern plastics. One more annoying thing about celluloid. It needs a very long time to cure after manufacture. In rod form, it can take several years to fully cure and reach a stable dimension. 15 years ago, an Italian pen company, Aurora, released a series of pens in celluloid but did not give the material enough time to cure. A year after making and selling the pens, the parts all started to shrink and the pens had to be replaced.
Check out this video to see how celluloid acetate is made (it is basically the same for celluloid nitrate other than the base chemistry).
Video . Mazzucchelli 1849