Wanted: black sardonyx chunks

Onyx is black to black and white banded chalcedony (cryptocrystaline to microcrystaline quartz).  Although the name is often incorrectly applied to banded calcite (calcium carbonate), which if course is a completely unrelated stone.

Sard is brown chalcedony.

Sardonyx is reddish or orangish brown banded chalcedony.

I find true onyx here in Southern Nevada and know a location in Southern California loaded with sard. Never come across sardonyx though.

There is a black jasper, which of course is a type of chalcedony, but is opaque as opposed to the onyx version.  I would be really surprised if Rock Shed was selling real onyx, which again is the black to black and white chalcedony, for tumbling considering the rarity and thus high cost.  I have only seen real onyx sold for high end jewelry. 

As an example this site the faceted and cabbed onyx goes for $1 per carat, which is $452.00 a pound.  Not sure if it is natural or treated sard as they do not specify, but do mention the stones are often colored.  www.gemselect.com/onyx/onyx.php

Seen some stuff on Etsy they claim is onyx.  Most looks like black jasper, and one site I looked at, it did not look like an form of chalcedony.

I looked up the Rock Shed "sardonyx" and this is the description. 

"Sardonyx is a form of Chalcedony. Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganite. Chalcedony has a waxy luster, and may be semitransparent or translucent. It can assume a wide range of colors, but those most commonly seen are white to gray, grayish-blue or a shade of brown ranging from pale to nearly black."

What they have is chalcedony, but not sardonyx.  Sardonyx, just like sard and onyx, are very specific types of chalcedony and none of these come in a range of colors.  Specific chalcedonies can have different names such as chrysoprase, tiger's eye, "agatized wood", gem silica, aventurine, bloodstone, chert/flint, etc. Or they may simply be named after their color such as purple chalcedony, blue chalcedony, green chalcedony, yellow chalcedony, pink  chalcedony, etc.   Sardonyx, as well as sard and onyx though are very specific chalcedonies.  Sardonyx only being translucent reddish brown or orangish brown with banding. Sard being a translucent brown chalcedony, and onyx being a translucent solid black or black and white banded chalcedony. Therefore, what the Rock Shed is selling as "sardonyx" is not sardonyx at all.

Getting back to chalcedony in general, the chalcedonies are common chalcedony, agates, jaspers and chert/flint. 

Common chalcedonies are generally clear to translucent.  Agates are defined as clear to translucent chalcedonies that have multicolored curved bands. Therefore, many stones often referred to as "agates" such as common chalcdedonies, "snakeskin agate", "moss agate", "plume agate", "dendritic agate", "waterline agates", "plasma agate", "agatized wood", etc. are not really agates by geological definition.  Jaspers are opaque forms of chalcedony, as is chert/flint.

The other difference between chert/flint and all the other chalcedonies is the silica source.  All chalcedonies are formed as the dehydration product of opals, which is primarily amorphous (lacking crystalline structure) silica gel.  In short, opal starts as a concentrated silica solution. As the molecules move closer together, water gets squeezed out. When the water content reaches at least 23% the result is opal, which is not a true solid, it is a hard gel with the molecules still in movement. This plays a big role in crazing of opal, that I am pretty sure I explained in a previous post here.  Anyway, the water content will continue to drop over time as the molecules continue to move closer and closer together squeezing out more water.  As the water content of the opal drops below 3%, the opal starts to crystallize in to some form of chalcedony, which is a true solid.  Chert/flint is formed from opal derived from biogenic sources of silica such as diatoms, radiolarians and sponges, and is a sedimentary form of chalcedony.  Other forms of chalcedony form from inorganic sources of silica such as clays and sandstone.

Unfortunately, in the rock trade, many stones are mislabeled leading to a lot of confusion.  Many stones that are not really agates being sold as "agates" as explained above.  "Silicated chrysocolla", which is actually a chalcedony stained with copper salts, not chrysocolla.  Fire agates, which are chalcedonies stained with iron salts. "Bumblebee jasper", which is not a jasper at all, but rather a calcite.  "Rainforest jasper", which is not jasper either, but rather a rhyolite.  Satin spar, which is the fibrous and translucent to opaque version of gypsum, is often mislabeled as the more rare and more expensive "selenite". Selenite though is the clear, non-fibrous form of gypsum.  Etc.

May sound nitpicky, but there is a reason for different, proper identification of stones. Diamonds, graphite and coal are all different forms of carbon. That does not make it alright to sell graphite and coal as black diamonds.  

By the same token, in my opinion, common chalcedony should not be sold as the more rare and more expensive sardonyx as it is not only misleading, it also leads to more confusion in stone identification.  A great example of this is how I constantly see people calling any copper mineral or copper ore  they cannot identify as "chrysocolla".  I  have even seen malachite and turquoise referred to as "chrysocolla".  Watching this on various rock boards you would think chrysocolla is so common that we could pave our streets with it.  The fact is that chrysocolla, which is a very soft hydrated copper silicate, is rather rare.  Newbies get improperly taught though to ID any copper mineral or ore they cannot identify as "chrysocolla", then they teach more newbies this and the cycle continues.