Dendritic Moss Opal / Opalite / Opalized

by georgeingraham on September 19, 2009

Recently received several slabs of what I believe to be a dendritic Moss opal.  Not entirely sure if it is an opal, opalized, or a opalite. I have read that opalite is an opalized volcanic ash than can occur with common opal and agate. It can be dendritic.

Opalite is very hard and can have a wet appearance upon breaking creating the illusion of an opal. I don’t really see this wet appearance in the darker, and not at all in the white.

Common opal fractures into tiny pieces/crumbs upon hitting, and opalite does not. This seems to better fit the description. When I break a slab with a hammer the white (softer) does not really break into very small pieces, where as the darker (harder) areas do.

Here is as best a couple of pics of the breaks on each. I am guessing the white areas to be about a 5 and the dark to be 6 or better.

Photobucket Photobucket

It is not uncommon for common opal to be associated with moss and even plume agate. It can be found in white, tan, brown, green and peach.

Here are a couple of slab pics.

2 30

Then a couple of cab pics.

24 A 36

The harder dark material took a great polish using tin oxide. The softer white colored material did not.  At the time of this writing have not yet tried another polish. Thinking some Holy Cow. Other than that, I am not familiar with a better polish. Prior to polishing these came off a worn 600 silicon carbide belt.

One thing I noticed while cutting preforms was that as I got towards the end of the cut, maybe an inch or sometimes even two, the slab would break. Not because of any fractures. It would consistently break in line with the direction of the remaining cut. I had to cut very slowly as I moved towards the end of the cuts. Can’t say I have ever experienced this with any other material before. Just sort of odd…

The material comes from Northwestern Missouri. I have read of an Ogallala Formation through that area. The Ogallala opal can be of these same colors as well as colorless, and cherty. Some of it is called moss opal because it contains a manganese oxide impurity that forms these dark, branching deposits that look like small mosses in the opal.

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