Pyrope is a garnet just like almandine and several others.
Easiest way to see what makes the difference between garnet minerals is to take a look at the table below:
Pyrope is a magnesium garnet, almandine is an iron garnet, and spessartine is a manganese garnet. Note that most natural minerals are mixtures of the three (dominantly mixtures between pyrope and almandine or almandine and spessartine). Absolutely pure endmembers don’t exist in nature anyway but solid solutions in garnets are really common. Actually, pure endmember is really rare. Most commonly up to 70% of the sites in crystal lattice that are shared between Fe, Mg, and Mn are occupied by Mg in pyrope. The composition of garnet may be expressed the following way: Py62 Al35 Sp3. Mineral with such a composition is called pyrope. To make things more complicated, other chemical elements may also enter the crystal structure. Chromium, for example, is especially common.
Pyrope crystals within an ultramafic rock wehrlite (peridotite). Bright green mineral is chromian diopside, yellow is olivine. Åheim, Norway. Width of view 20 cm.
Another sample of peridotite with lots of purple pyrope. Åheim, Norway. Width of view 25 cm.
Pyrope demonstrating its hardness and durability as it stands out while olivine and pyroxene are not nearly as resistant to weathering. Width of sample 11 cm. Åheim, Norway.
These three minerals are collectively called pyralspites (pyrope + almandine + spessartine). We need this term because there are more garnets than these three mentioned above.The rest of the garnet group minerals are somewhat different. They are called ugradites (uvarovite + grossular + andradite). There are no continuous solid solution between the two groups. Therefore, I will not pay attention to the ugrandite group now. There are even more garnet varieties possible but these are really rare and have no geological significance.
All pyralspite minerals are shade of red — light pink, yellowish red, red, purple, very dark red (almost black) are the most common possibilities. The color depends on the grain size. They all tend to be lighter in sand and darker as large grains (this is universal among minerals). But the color also depends on the composition. Pyrope is the darkest of the three. It is usually dark red, purple, or almost black.
Unfortunately, color only is not a reliable guide. Perhaps more useful is to know from what type of rocks are these minerals coming from. Almandine is the most common of the three and it comes mostly from metamorphic rocks (garnet schist, garnet amphibolite). Spessartine comes from igneous rocks like granite and pegmatite. Pyrope comes from ultramafic rocks (peridotite, pyroxenite, kimberlite, and serpentinites derived from them) or ultrahigh pressure metamorphic rock eclogite.
Pyrope is a semi-precious gemstone partly because of its beautiful dark red color and partly because it is much rarer than almandine. It is rare, of course, on the upper part of the crust. The rocks containing pyrope are not rare at all deep below.
Pyrope is one of the index minerals in diamond prospecting. It does occur together with diamond in kimberlite pipes. It is not a typical kimberlite mineral but kimberlite pipes often contain pyrope-bearing blocks as xenoliths. These diamond containing pipes are small and hard to find. The grains found in river sand might indicate that ultramafic rocks could be somewhere near. Prospectors just need to go upstream and take samples until they find no pyrope anymore. Then, if they are lucky, diamond bearing kimberlite pipe could be nearby.