Quartz is composed of silicon dioxide. Atoms of SiO2 linked inside the quartz crystal form a tetrahedron that share an oxygen molecule between them. Hence, any one particular molecule of crystalline quartz has the formula SiO4. The varieties of quartz can be organized by color, as follows: purple amethyst, yellow citrine, blue dumortierite, milky quartz, rosy quartz, brown smoky quartz, and green prasiolite. While blue quartz contains material other than dumortierite, the opaque and transparent smoky quartz can be as dark as black. Aluminum inside the smoky quartz crystal can turn from transparent to translucent when irradiated, so the smoky variety of quartz may be produced in a laboratory.
Because of its durability, quartz has a history of uses both for jewelry as well as for stone carvings. The family of quartz include varieties of semi-precious gemstones because the abundance of quartz crystals is only second to the abundance of feldspar. Durability and abundance make quartz an ideal gemstone for public display. Quartz crystal has the ideal shape of two prisms terminated at both ends, however, quartz crystals are often found twinned or inter-grown with other quartz crystals. Geodes with quartz are typically druse, crystals lining a void. Quartz crystals do not have cleavage hence they do not fracture along a plane. Since ancient times, the fact that iron could fracture some stones and not others raised confusion because the firmness of a stone could yield by force to iron like the whetstone of Armenia. Others instead were observed to be carved by other stones such as the stones used for seals.
"And such stones are harder. In much the same way it is peculiar that a stone cannot be cut at all with iron, and yet iron too, which is harder than stone, cuts which are firmer and stronger." (translator note by Earle Caley from Theophrastus, On Stones; page 54)
Today, the term "rock crystal" may be used to refer to clear quartz, while in the past, the ancient Greeks used "icey cold" to refer to clear quartz because they believed quartz to be super cooled ice. Amid those who believed in super cooled ice was the writer and philosopher Theophrates, a student of Aristotle. In the 14th century, the German word "Quarz" was derived from the Polish word "kwardy" and the Czech word "tvrdy", meaning "hard". Thus, it's of little surprise that stone quartz carvings were discovered in Eurasia. The Lothair Crystal at the British Museum is a hard stone carving depicting the biblical story of Susanna and dated to 855 AD. The dating for this art came from the inscription LOTHARIVS REX FRANCORVM IVSSIT in reference to Lothair II who styled himself as "rex". The Lothair crystal has since been cited as an incidence of Carolingian art.
Quartz during the Middle Ages was also known as Iris or "rainbowstone" because of its prismatic quality of casting light onto a wall. According to Ragiel, Iris engraved with a man in armor has the power to protect the wearer. Just a transparent quartz crystal similar to rock crystal, Iris was found between the Rhine River and Treves, Germany. Transparent gypsum might be mistaken for Iris. Pliny had made the hypothesis that the stone's hexagonal form was the result of being closely packed near other stones even though the surrounding stones were round; thus, the form of the quartz crystal had seemed similar to a honeycomb since the edges were also rounded. Today, the vander waal forces inside the crystal hold it together, and the positions of the atoms determine its shape. The rainbowstone is described by Albertus Magnus as being dry, of considerable hardness, and produced out of red clay, where the moisture of the clay is attacked by the quality of its being dry. By holding part of the stone indoors while other parts are shaded, the rainbow reflection given by this stone refers to its name being Iris.
Quartz crystals have traditional uses in magic and spirit mediation. Some believe thoughts and impressions are stored inside the crystalline lattice and, similar to a mojo bag, every crystal has a personality. The idea that clear quartz can carry-out the desires of the magician shouldn't depend on the form of the quartz crystal because each individual crystal has the same form according to Steno's Law. Thus, in my opinion, clusters of quartz crystals when placed inside a pyramid or at the vertices for magic are the equal of rock crystals at the same locations. Clusters will make a good replacement with this configuration.
I say this because the pyramid can be associated with the sacred geometry of the tetrahedron. The placement of quartz either at the vertices or the center of the tetrahedron symbolize the principle of infinite space. Clusters of crystals being fused together by the process of growth can either face in to the center or being the center itself, the four having equal value as one. So, it is a mater of utility. One large rock crystal placed inside a pyramid may offer the spirit an inspiration of being at the center-stage, whereas the clusters placed outside the pyramid may highlight the emptiness of the space within creating a sense of urgency or importance.