Synthetic pen materials are offered by a wide variety of manufacturers, including some local folks who like to play with the chemistry and additives themselves. The core of most of these is two part mixture of liquid plastics which combine to form a super-hard block of (usually) acrylic/polyester.
If you press me to tell you what that means, I'm going to give two answers. One is: Chicken Wire. As in, the molecules that make up these materials tend to look like this.
The other answer is: It's much like your countertop material, or the handle of your toothbrush. Whether these are clear or opaque, sparkly or low-key, whether they have a metallic sheen, iridescent swirls, or clouds of color are decisions left entirely to the manufacturer. The chemistry and techniques of composites, plastics and polymers are incredibly well researched, since so much of modern manufacturing relies upon them.
There are also products which combine natural and synthetic materials to good effect. Plant materials, minerals, wood with voids and spaces in it, and even seashells are popular inclusions in the acrylics, and the resulting made items can be spectacular.
The composites can include novelty materials (glow in the dark pieces, coffee beans, pieces of dry pasta), or memento items, such as when you find an insect encased in clear Lucite blocks. It is also possible to make objects from crushed minerals and composite resins, which means you can have a pen made of malachite without having to pay the cost of having your pen made by a lapidarist.
... but the results are well worth the pain of decision-making!