Opal is a fascinating mineraloid delight for many mineral and gem collectors. It comes in a variety of colors. There is common Opal and precious opal. Let’s take a deeper look at this quirky member of the silica family.
What is Opal
You may see Opal referred to as a mineral. However it is a mineraloid with no ordered atomic structure composed of hydrated silica (SiO2.nH2O). Composed of spherical silica aggregates, it typically contains 6-10% water.
Precious Opal
Opal is considered precious when it displays iridescence, or play of color. Play of color is happens when the silica microspheres making up the stone are tightly packed in a regular order and diffract light. So when you turn a piece of precious opal in the light the refraction changes causing color change. Some colors are caused by small impurities. For example fire opal contains small amounts of iron oxides.
Common Opal
The common variety can be found in a variety of colors, though without flash and play of color. This material is opaque and rarely transparent. It is readily found in many parts of the world.
Here in Union County, New Mexico, milk opal (white) is the most common. It is often found with Manganese dendrites (ancient Greek for tree). Mostly it can be found along roads that have or once had caliche for road base. It’s not everywhere due to the caliche being sourced from a number of different pits being when they built and maintain the roads. I’ve hounded in our caliche pit on the ranch and have yet to find any. However I know several locations both on and off the ranch to find it.
Properties of Common Opal
Support Willett Creek Agate Content
If you enjoy or learn from my content, there are a couple of ways to support my work.
Probably the easiest way you can support my work, is to visit the shop for merch, agates, common opal, and more.
I also offer memberships or you can make a one-time donation to buy me a coffee rock here. In exchange for your support, I offer a variety of membership levels and perks.