Dendritic agate is a translucent, colorless to whitish-gray variety of chalcedony quartz, easily distinguished by its distinct tree- or fern-like markings known as 'dendrites', which are most often brown to black in color. It is considered to be one of the rarer and more sought-after varieties of agate stone, although it is technically not a 'true agate' since it lacks the typical banding of other agate stones.
'Dendrites', a Greek word for 'tree-like', are inclusions that appear organic due to their plant-like structure, but they are actually inorganic traces of iron or manganese. The iron and manganese ions gain access to the forming stone as weathering solutions from neighboring rock. The impurities crystallize inside the agate in branching formations seeping through fine surface fractures. In some cases, dendritic inclusions may form in spotted clusters rather than branching structures; these are sometimes referred to as 'mosquito stones'.
Agates were used as gemstones by ancient Egyptians over 3,000 years ago, making them one of the oldest gemstones mentioned in recorded history. In Russia, dendritic agate was prized as a stone of prosperity, good health and longevity. It is associated with the protection of the elements and is believed to aid communication with all things in nature.
Dendritic agate encourages healing, restoration and creativity. It is said to improve self-esteem and emotional balance. It is a stone of protection and since biblical times, has been worn and used to protect against storms. It was highly valued as talismans and amulets.
Dendritic Agate vs. Dendritic Opal ? This is taken from gemsociety.org written by "Dihusky"
Chalcedony could be considered an ‘umbrella’ name, Agate is a type of Chalcedony as is Jasper, Carnelian, Chrysophase, Onyx and many others. All are polycrystalline structures of quartz and the random banding defines an Agate, whereas straight and parallel banding with white becomes Onyx, 'dendritic refers to the inclusions. Opal on the other hand is quite different depending on it origin. The only thing they have in common is that all are silicates.
Gem Opal as found in Australia and a few other locations around the world is Mineraloid/Amorphous and does not have a crystalline structure, instead it is made of billions of tiny silica spheres in a mixture of SiO2 and water. This Opal forms in a different way, its formation is via the Silicon dioxide ‘soup’ entering fractures in the host material and solidifying over millions of years.
Chalcedony has a hardness of around 7 Moh whereas Opal is 5.5-6 Moh.