Macerals are defined by both their color/reflectance and morphology. The two cannot really be separated. It is not always easy to decide which category a maceral is in; even coal petrographers don't always agree. (A coal petrographer is someone who systematizes and classifies coal. Often he or she has a strong background in geology.)

Its not hard to figure out why some macerals have the names they do. Many take the name of the material from which they were derived: sporinite from spores, alginite from algae. However, in some cases its not obvious. Liptinite is derived from the word lipid, which are highly aliphatic materials. i.e. they have a chemical composition similar to waxes or oils. Vitrinite is derived from the word vitreous, because of its glossy appearance in a chunk of fresh coal. Inertinite is derived from inert, because some of these
macerals don't react or change very much when heated.

Sidelight: Who came up with the idea of macerals? Well, here is another surprise. The first person to come up with the concept was Marie Stopes. The system we now use is more elaborate then she envisioned, but the origin is attributed to her. She was a geologist by profession. The surprising part is she is remembered
more for her other escapades. She was an activist for feminine rights in the thirties and forties, and an advocate of birth control. She had a very colorful history, and even spent a fair time in jail because of her militant dedication to her unpopular and radical views for that time.

One feature that is not clear from the Table is that the reflectance and color of a coal changes as its rank increases. In reflected light the vitrinite becomes much brighter. However, at the same time the liptinites, which appear darker than vitrinites in low rank coals, increase their reflectance even more rapidly. Inertinites, which are brighter than vitrinite don't change their reflectance very much.
So in very high rank coals (medium volatile bituminous and above, we can't see the different macerals without very special techniques.

Properties of Some Macerals

In addition to their appearance, macerals have many other different physical and chemical properties. Again though, the properties of macerals change as a function of rank or maturation. They cannot be considered as a single molecular species with a well defined chemical structure. Thus, it is difficult to summarize their properties. In addition, even an approximate chemical makeup of many macerals is unknown. The following Table gives you an idea of some differences for the more common macerals of high volatile bituminous rank.

A Table of properties such as this can be misleading. The ranges given for the maceral properties are affected by several factors. One is the rank of the coal. Most macerals will show a decrease in
H/C, O/C, and N/C ratios as coal rank increases. Density of many macerals goes through an even more complicated change. However, there are also variations in chemistry for macerals of the same rank.
This can be caused by such factors as the conditions of the original peat, and the variation in chemistry of the original plant part. For instance, it is well known that pollens and spores have different
chemical resistances and therefore different structures.

From the Table, coal contains large amounts of aromatic material. In high volatile bituminous coals most of the aromatics moieties are substituted benzenes and naphtalenes containing macromolecules. These macromolecules may have molecular weights in the range of several thousand daltons, but this is not
completely established. As the rank increases the size of the aromatics tends to increase. At the right is a model of bituminous coal. However, there is still much speculation on coal structure.

This quite specific structure of coal does bother some coal scientists, and with our present knowledge of coal chemistry, they prefer a less specific picture of coal similar to that shown below. The numbers represent the number of carbons in a cluster (mostly aromatic). The single lines represent connections between clusters.
Although this structure is based on the properties of a higher rank coal than the previous structure you can see that some relationships are similar. For instance, the idea of linked compact aromatic clusters and the presence of heteroatomes. Nevertheless, this latter representation of a high rank coal does point out that there are many gaps in our understanding of coal's physical and chemical structure. Part of the reason for this uncertainty is that all our evidence for the detailed macromolecular structure of coal is based on both thermal and chemical degradation methods, which themselves have uncertainties and complications in their reaction mechanisms.

Information provided by: http://chemistry.anl.gov