Moss opal and dendritic opal

[janeyjane]

Could someone please tell me the difference between moss opal and dentritic opal?
Thanks.

 

[VapidLapid]

I am sure there will be a lot of variability in responses, but I think any hydrated silicate material can be termed opal. This can be transparent like jelly or other gem opals or even opaque like an opalized jasper. I have seen opalized jaspers with manganese dendrites. I think that moss in such stones always implies some degree of transparency or at lest translucency in the parent stone, and can allow quite a variability in the nature of the included, mossy material.

 

[janeyjane]

Thank you for your answer, I am very confused about this.
So could the same opal be called either "moss opal" or "dendritic opal"?
Could a "dendritic agate" also be called a "moss agate"?
Does dendritic simply mean that the inclusions are "tree-like" or that the inclusions are iron and manganese? Does a moss opal only have hornblende inclusions or can they be iron and manganese?

 

[VapidLapid]

From Gemselect:

Because of its dendritic inclusions, dendritic agate bears some similarities to moss opal. But dendritic agate is not called moss agate. In fact there is a another gemstone called moss agate, which has inclusions of green hornblende in moss-like patterns.

From wikipedia:

Included matter of a green, golden, red, black or other color or combinations embedded in the chalcedony and disposed in filaments and other forms suggestive of vegetable growth, gives rise to dendritic or moss agate. Dendritic agates have fern like patterns in them formed due to the presence of manganese and iron oxides. Other types of included matter deposited during agate-building include sagenitic growths (radial mineral crystals) and chunks of entrapped detritus (such as sand, ash, or mud). Occasionally agate fills a void left by decomposed vegetative material such as a tree limb or root and is called limb cast agate due to its appearance.

I think the best discussion comes from here though it doesn't answer your question specifically
http://jewellerstrade.com.au/2012/fractals-at-their-finest-dendritic-opal/::

Australian Dendritic Opal is typically a black and white ornamental gemstone containing areas of opaque white common opal, translucent grey to colourless chalcedony and picturesque black to brown dendritic inclusions of manganese oxides. Although it is often called “Merlinite” in the new-age scene it is however the dendrites that have made this stone popular, not Merlin.

Dendritic is the term used to describe the formation in which certain minerals crystallise; fine, fern-like branches much like the structure of frost and snowflakes. Dendrites are, by definition, a fractal – a “self-similar” pattern, meaning they are the same either near or far, an ever-evolving pattern of repetition and growth. Fractals are fascinating for more than just the dendrite enthusiast. Mathematicians and scientists have studied their attributes to further their knowledge of geometry, metallurgy, computer technologies, neurosciences, space and time. In the gemmological world dendrites are manganese oxides (MnO2) and form in fine fractures and fissures through the crystallisation of manganese rich solutions from surrounding weathered rock. In the jewellery world they’re simply beautiful and can turn a piece of common opal into something of spectacular beauty and intrigue.

Most people will look at dendritic opal and say “that’s not opal!” but our perception of opal has been skewed by our nation’s abundance of “precious opal” and the magnificent ‘play-of-colour’ it exhibits. Common opal (also referred to as ‘potch’) is precious opal’s not-so-extravagant cousin. Both are silicon dioxide (SiO2) with varying amounts of water (H2O), however the key ingredient of orderly, symmetrical micro-structure of silica spheres that are required to produce flares of spectral light is not present in common opal. Instead they are varying sizes and randomly arranged. With a hardness of 7, dendritic opal is a good stone for use in jewellery although the water content means extreme temperatures can negatively affect the stone.

 

[janeyjane]

Thank you so much for the useful and comprehensive information! I guess there is still some question regarding the term "dendritic" when it comes to the make-up of the inclusions. I'm guessing that "dendritic" is used as an adjective to refer to the appearance of the inclusions rather than what they are made of.