What is a Geode?
Imagine picking up a very plain dull looking rock, it seems solid but strangely light weight. Crack open this rock and what greets your eyes is a hollow centre full of glistening gems, this is a Geode. Hollow rocks like this are various sizes; typically between25mm and 30mm across, either oval like a rugby ball or maybe a squashed shaped shape like a pasty. They are most often of volcanic origin, having the classic crystalline centres of all the Geodes; sometimes they also have some other features more often seen in the solid Agates, many are just partially filled.
Ocos or Ochos Geodes
One of the most spectacular occurrences of Geodes are those from the Serra Geral Formation, Rio Grande do Sul, Brazil. Another famous location of Dark and Reddish soil Deposits is the Oco (Ocho) Geode Deposits from Tres Pinheiros, Fountoura Xavier, Rio Grande do Sul, South Region, Brazil. They have amazing crystal lined hollow centres, lined with clear Quartz crystals that sparkle like cut diamonds in the sunlight. These Oco Geodes, sometimes spelt Ochos are named such because Oco is the Portuguese word fro a hollow stone. They were found abundance in local ploughed fields for many decades, broken open by the plough.
Ocos are found in the weathered volcanic rocks of the area and are between 127 and 138 million years old, dated from the surrounding rock. The host rock is amygdaloidal basalt that has been weathered to produce the dark reddish soil (Cascalho) found in the pampas grasslands of Southern Brazil. Ocos are discovered scattered amongst the hills in an area called the Three Pines Region about 80 miles northeast of Soledade. Only half of all the Ocos discovered are later found to be actually hollow. These stones often have a classic rope like texture and when removed from the weathered rock; they also have a similar textured interior, when sliced open. After the rocks have been transported, sometimes by mule, to the town of Solidad, they are then cut in half and polished.
Minerals and Geodes
Apart from the Quartz Crystals that lined the cavities, some Geodes also contain other secondary minerals, often they are Black Oxides of either Manganese or Iron, Goethite is the most common mineral after Quartz, this mineral is found as a fine acicular needles. Amongst the other crystal treasures are Rare minerals such as Hollandite, Cryptomelane, Rancieite, Bernessite, Beidellite, Ramsdellite, and Todorokite all are dark Brown to Black and difficult to identify. Other minerals to be found include Haematite, Siderite, Chalcedony and Calcite.
Plumes and Patterns
Some Ocos also have remarkable feather like patterns, clearly seen on the cut and polished surfaces, plain stripes can also be seen on some of the more recently polished externally to show off the amazing patterns and swirls of light stripes on the dark background. Small examples (less than 25mm) of Ocos are often cut into thin flat slices of the Geode material, primarily for use in the Jewellery trade. These slices are then electroplated on their outer edges with silver or gold and made into pendants.
Water and Oil
Some of these Geodes have hollow centres that contain fluids; these Geodes are then called Enhidros, sometimes spelt Enhydros, containing fluids that are presumed to have existed since their original formation. These Chalcedony treasures are very skilfully cut and polished to display the enclosed waters, and sometimes trapped air bubble. These Fluids can be seen to move around inside the partially full cavities. There are also unconfirmed reports of oil being seen oozing from freshly broken Geodes.
Additional Colour
Many Geodes display areas of Red or Yellow colour due to various Iron Oxides that are derived from the weathering of the surrounding Red or Yellow soil. These colours look vey striking when set against the otherwise White crystals, or pale Geode surfaces, other Oxide minerals are often found alongside. The Geodes can also contain other varieties of Quartz Crystals, such as Pink Amethyst; Brown Smokey Quartz, or Yellow Citrine. The outer shell of some Geodes can also Rarely be very thin and translucent, these are considered to be highly collectable.
Calcite in other Geodes
Some other Geodes can contain crystals of Calcite, either as flat plates or the well known Scalenohedral crystals. Calcite from natural organisms are also found to be replaced by Silica rich Chalcedony, such examples are seen in the Coral Pseudomorphs of Tampa Bay, Florida, USA. These Fossil Corals usually have a classic external Fossil with an internal Botryoidal Chalcedony.
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