True, but among the minerals with cubic crystal structure it is not unusual for them to be found as crystals that are perfect regular or semiregular polyhedra, with a shape characteristic for the mineral, for instance octahedron (e.g. spinel, diamond), rhombic dodecahedron (e.g. garnet) or cube (e.g. pyrite).
I suppose that the crystals from the picture are of pyrite, which frequently looks like this.
In the antiquity, when what are now called diamonds (the Romans and the Greeks called them "Indian adamants", because they were first encountered by Europeans during the expedition in India of Alexander the Great; "adamant" meant something else in Europe) were very difficult to cut and polish, they were normally used as gems in their natural shape of regular octahedra.
Cutting diamonds from their natural octahedral shape into polyhedra with more facets, e.g. brilliant, was invented much later.
Regarding etymology, for many centuries the substances that are now called "sulfides" were called "pyrites", after the "iron pyrite" i.e. the iron (II) disulfide, which is the most abundant sulfide mineral.
At the end of the 18th century, Lavoisier together with a few other French chemists have created the modern systematic chemical nomenclature, so the old term "pyrite" was replaced by "sulfide" (like also "vitriol" was replaced with "sulfate").
For who does not know, "pyrite" comes from "fire", i.e. from the pronunciation in Ancient Greek of the corresponding word that was cognate with English "fire" (Ancient Greek or Latin "p" corresponds with English "f").
Striking pyrite produces sparks, which can be used to start a fire.
> Ancient Greek or Latin "p" corresponds with English "f"
Similarly, ancient Greek "p" corresponds with modern Greek "f", and ancient Greek "b" corresponds with modern Greek "v".
We may have done it first, but the sound change is pretty common. "B" -> "v" is arguably in process in Spanish. Something similar had already happened in Latin; compare Latin "frater" to English "brother" or Latin "fero" to Greek "phero".
Especially since it's an exception that breaks the rule that straight lines are not found in nature. Not only is it a straight line, but a cube. They just look unnatural. Very cool stuff
I suppose that the crystals from the picture are of pyrite, which frequently looks like this.
In the antiquity, when what are now called diamonds (the Romans and the Greeks called them "Indian adamants", because they were first encountered by Europeans during the expedition in India of Alexander the Great; "adamant" meant something else in Europe) were very difficult to cut and polish, they were normally used as gems in their natural shape of regular octahedra.
Cutting diamonds from their natural octahedral shape into polyhedra with more facets, e.g. brilliant, was invented much later.