I embarked on writing this blog with a straightforward question in mind: what sets sunstone apart from moonstone? It's a query that had eluded me for some time, and despite extensive immersion in the mineral world, I had never found a direct answer. After thorough research, the distinction between sunstone and moonstone, ultimately comes down to a visual distinction of the inclusions and the optical phenomenon they create. If no inclusions are present we delve into the chemical composition and crystalline structure.

To unravel this enigma, we must delve into the world of feldspars, which constitute a staggering 60% of the Earth's crust and found in lunar rocks brought back from the moon by various Apollo missions. While many recognize feldspars by their common names like labradorite, moonstone, sunstone, and amazonite, there exist numerous other varieties worth exploring. Understanding these distinctions not only satisfies our curiosity but also sheds light on the age-old debate of sunstone versus moonstone.

Let's dive into one of the most fascinating aspects of feldspars: their formation. While some feldspars are created through metamorphic processes, where existing rocks undergo changes without melting, the most captivating method is through melting. During the igneous process, crystals form as the molten material cools, akin to the formation of chunks in tapioca pudding. This process, scientifically termed "nucleation," begins with the formation of tiny clusters of atoms, ions, or molecules within the parent material. These clusters, known as nuclei, serve as the initial building blocks for the new crystalline phase. Nucleation occurs within a material undergoing a phase transition, such as solidification from a liquid or gas. In the context of feldspar formation, nucleation takes place within cooling magma or molten rock. As the temperature drops, atoms or ions within the molten material organize themselves into ordered structures, leading to the formation of nuclei.

As feldspar crystals take shape, they fall into one of two categories: potassium-based or sodium and calcium-based (plagioclase). Regardless of their composition, feldspars are invariably composed of aluminum, silicon, and oxygen—their building blocks. Chemistry, while often dismissed as dull, plays a crucial role in distinguishing between sunstones and moonstones, forming one of the key concepts of this discussion: the Potassium Group or the Plagioclase group.

Refer to the chart below for a clearer understanding of feldspar classification.

Potassium Feldspar Group

Lets get the easy ones out of the way:

Microcline and Amazonite

Microcline can be described as essentially white amazonite. The distinct green hue of amazonite arises from trace quantities of lead, iron, or organic compounds integrated into its crystal structure.

We are going to skip Andularia and Sanidine because 95% of you will never encounter them in your life.

Orthoclase Feldspar

Under the Potassium feldspar group, we have the orthoclase variety which is further defined by having a monoclinic crystalline structure. If orthoclase feldspar has optical properties- we use them to determine whether the stone is sunstone or moonstone.

Sunstones:

Optical Properties: Sunstones are characterized by their aventurescent shimmer, caused by reflective plate-like inclusions of minerals such as hematite or goethite within the crystal structure. This shimmering effect is often visible under direct light, particularly sunlight.

Appearance:Sunstones typically exhibit warm tones such as golden, orange, or reddish-brown, often with a sparkling or glittering appearance under direct light.

Moonstones:

Optical Properties: Moonstones, on the other hand, display an adularescent glow, which appears as a billowy or milky sheen that moves across the surface of the stone when viewed from different angles. This effect is caused by light scattering between thin, alternating layers of orthoclase and albite within the crystal structure.

Appearance: Moonstones are often whitish, bluish, or silvery in color, with a more subdued and mystical appearance due to their adularescent glow.

If our orthoclase feldspar has no optical properties, there is no way to determine if it is sunstone or moonstone, so rather than living with an indiscernible answer- we should simply call it orthoclase feldspar and call it a day. However, market demand plays crucial roles in the final determination.

Plagioclase Feldspar Group

Lets get the easy ones out of the way:

Peristerite:

Peristerite emerges during the cooling and solidification of magma, particularly within igneous rock formations. This process gives rise to exsolution lamellae, delicate layers or bands of one mineral that segregate from a host mineral during crystallization. Within peristerite, these lamellae manifest as intricate layers of albite and orthoclase within the crystal structure, imparting the distinctive bluish-white adularescence that sets peristerite apart from other feldspar varieties. Despite being two different varieties of Feldspar in one, it has been grouped under the Plagioclase group because of its sodium and calcium content.

Labradorite:

It's important to note that exsolution lamellae are not the primary cause of labradorite's labradorescence.
Labradorite's labradorescence primarily arises from the diffraction of light within the crystal lattice due to structural features, such as twinning or lattice distortion. This diffraction creates the vibrant play of colors characteristic of labradorite.

We are going to skip Andesine and Bytownite because 95% of you will never encounter them in your life.

Albite & Plagioclase

Albite is differentiated from Oligoclase in that albite is a distinct subgroup characterized by its triclinic crystal structure and sodium-rich composition.
This means that an albite moonstone is still an albite moonstone scientifically whether it has optical phenomenon or not- though, the same standards in differentiating optical phenomenon in sunstone and moonstone still apply in the plagioclase feldspar group.

Oligoclase feldspar has less calcium and sodium than albite and though may require scientific instruments to distinguish the difference when no optical properties exist- the distinction is still identifiable.