65 million years in every stone
When you hold an opal, you are touching the result of a geological process that began in the Cretaceous period, approximately 65 to 70 million years ago. To understand what makes this gem so singular, you need to understand how it is born.
Silica, water, and time
Unlike most precious stones formed through crystallization under pressure, the opal is an amorphous mineral with no regular crystalline structure, composed of hydrated silicon dioxide with a water content ranging from 3% to 21%. Silica-rich water percolates through sedimentary rock layers, filling fissures and cavities. Over millions of years, the water evaporates slowly and the silica deposits in ultrathin spherical layers. These microscopic spheres, organized into quasi-regular structures, create the optical phenomenon of opalescence.
The secret of color: light diffraction
The play of colors in precious opals is not pigment. It is physics. Silica spheres of 150 to 400 nanometers act as a natural diffraction grating for visible light. As white light enters the stone, it decomposes into the visible spectrum, and each viewing angle reveals different combinations. Smaller spheres produce violet and blue; medium spheres produce green and yellow; larger spheres produce red and orange. Opals that display all spectrum colors are the most valued.
The geological context of Piaui
The opals of Pedro II are hosted in sedimentary rocks of the Parnaiba Basin, one of South America's largest intracratonic basins, associated with the Pedra de Fogo Formation. Hydrothermal cycles combined with the specific chemistry of local rocks created ideal conditions for high-purity silica precipitation, resulting in opals with exceptional color saturation that set them apart from deposits elsewhere in the world.
Why every opal is unique
The absence of crystalline structure means no two specimens are identical. Distribution of silica spheres, water content, trace minerals, and deposition patterns each contribute to an exclusive geological fingerprint. This radical uniqueness aligns philosophically with contemporary jewelry: a piece that cannot be replicated, an object that is genuinely irreproducible.
Care and stability
Because it contains water, the opal requires specific care. Sudden temperature and humidity changes can cause crazing, a network of micro-fractures that compromises the stone. Lower water content opals are more stable and better suited for daily wear. Always acquire opals of known provenance, cut by specialists who understand each stone's characteristics, and store them away from extreme heat and chemical products.
Source: GOMES, C.B. et al. As Opalas do Piaui [The Opals of Piaui]. Brasilia: CETEM/MCTI, 2025. Part of Fortes Jewellery's educational gemology series, based on scientific research by the Centro de Tecnologia Mineral of Brazil's Ministry of Science, Technology and Innovation.