O - Opal

Opal is a mineral that belongs to a large group of minerals known as the “Silica family”. Silica, known by its chemical formula as ' SiO₂ ', is a mineral compound of two atoms; Silicon and Oxygen. The most common forms of Silica in our environment are crystals of Quartz, or sand, and also window glass.

Opal is unique however as it also has in its formula a component of water or ‘ H₂O ’. The complete formula for opal is ' SiO₂. nH₂O '.

Opal is formed from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone, and carries this silica-rich solution into cracks and voids, caused by natural faults or decomposing fossils. As the water evaporates, it leaves behind a silica deposit. This cycle repeats over very long periods of time, and eventually opal is formed.

Structure. Opals therefore consist of small spheres of silica arranged in a regular pattern, with water between the spheres which cause light to break up into the colours of the spectrum - like a rainbow. This process is called 'opalescence'.

Different Colours. Larger spheres provide all colours, smaller ones only blues and greens. Opals that have a predominantly red colour are very rare as they only occur where larger silica spheres were deposited. White opals have delicate, pale colours on a lighter background. Black opals (very rare and valuable) have a dark background and colours ranging from brilliant red through to greens, blues and purples.

Video.  There is a short You Tube Video explaining what an Opal is.

This Cache

You will find this cache in a rock cairn - a pile of stones marking the location of the cache

This cache is part of the Opal Muster Tail established for the Queensland Outback Geocaching Muster (QOGM) 2021 Paroo - one of fifteen caches each of which contain part of the coordinates for the Mystery Cache "Opal Muster Tail Final"

Each cache contains a logbook for you to sign.  In addition, there are some "opals" in each cache which must be opened to discover which "opal" has the clue to the final.  Record your clues to discover the location of the cache "Opal Muster Tail Final".

The image below is how the original cairn for this cache was constructed. PLEASE RETURN IT TO THIS STATE AFTER YOU HAVE REPLACED THE CACHE.  

Keep the cache covered but away from the rocks Their summertime heat could deform the plastic.  Especially the lid - keep the rock cover ABOVE the cache so it is NOT touching the lid.  Thank you for helping prolong the life of this cache.

Happy Fossicking!