These three pictures may help.
Quite a few years ago I bought two rods of vintage jade celluloid rod stock at an antique show, and had a friend make a pen from it. One rod was an appropriate diameter for a cap, the other the right size for a barrel. They matched in color and detail, and suspect may have been made for Parker since I have a Duofold Jr cap that is a perfect match.
This is the barrel as it is now, and as both cap and barrel were when the pen was made at the end of 2004. Note the detail, white swirl and deep color. It has not changed at all.
A number of years later I noticed that some spots in the cap were losing the white, and were starting to go transparent. This continued to the point where I pulled the cap off of the pen and set it by itself as it was obvious that the celluloid was starting to decay. Fortunately my friend had made it to Parker specifications, and though it was made as a Depression era pen, dimensions and threads matched Parker standards, so a Duofold Jr blind cap and cap fit perfectly - it already had the correct Duofold/Thrift Time section since Parker used the same section in a number of pens including the Streamlined Duofold Jr and Thrift Time pens.
This is how the cap is today, and shows perfectly advancing decay of celluloid. Note the brightness of the material, the translucency approaching transparency of the celluloid near the threads, and the loss of the white color. If you see this in patches on celluloid, the breakdown has started and can not be stopped. It will gradually accelerate to the point where the celluloid falls apart. It is relatively uncommon in Parker celluloids. I have one red Thrift time pen that is showing the signs in the cap, but it isn't getting worse. It is quite common in Wahl Dorics. I have no idea why one piece has failed like this, while the other is just fine, showing no signs of decay. They were fine for decades, the cap material broke down after it was machined.
One more picture of decaying celluloid. This is a gray/red veined Sheaffer OS Balance cap. This celluloid is also starting to break down. This color in particular seems to be vulnerable to this damage. Note the crazing and cracks. While the decay is relatively slow, it will continue. Trying to solvent weld the material to stabilize it often exacerbates the problem and accelerates the decay.