I got it into my head to try to find something authoritative on
this, after a friend suggested that he has reason to think that it’s
not iridescence from thin layers of iron oxides, as is commonly
thought.
I have been amazingly unsuccessful, finding only copies of copies of
assumptions.
I expected to find at least a reference to high-power (if not
electron-) microscope studies, X-Ray crystallography studies, etc.
– but nothing!
(I did find a reference to an article in Rock & Gem from '82, I
think; haven’t tracked that one down yet.)
I think the cause of color is fire agate is still thought to be
inclusions of goethite or limonite causing an irridescent effect.
Perhaps you can ask your friend for THEIR sources in disputing it
and let us know.
I’m not an expert, but a peek into Liddicoat’s Handbook of Gem ID
says that fire agate is a chalcedony with a brown bodycolor and
strong iridescence from hematite or goethite coatings on mammillary
(or, if you prefer, botryoidal), chalcedony formations, which have
been covered by still more chalcedony. That falls in line with the
"iron theory."
Honestly, though, I doubt that it’s a theory at all. It would be
difficult to find a published study, but the at least,
has been published by GIA, and others - Webster, for one. These
noted gemological luminaries studied all gem materials with far more
than just X-ray crystallography and scanning electron microscope
studies, including Secondary Mass Ion Spectroscopy and other
spectroscopic equipment. I can’t come up with a published paper
about it…yet…I’m having trouble with GIA’s EBSCO library. But
thorough analyses have been done by more than one prominent
gemological laboratory, and they aren’t assumptions.
However, even the most advanced equipment can lead scientists
astray. For example, tigereye was long considered a pseudomorph, or
quartz replacement. Quite recently, it has been discovered that the
quartz hasn’t actually replaced the crocidolite. Newer, more
technological equipment has allowed this new discovery. Perhaps your
friend is on to something.
It would be very interesting to know why your friend has reason to
believe that fire agate’s iridescence isn’t caused by the materials
that are currently believed responsible for the phenomenon. If he’s
right, he will go down in history. Will you let us in, or do we wait
for the press release?
"Opaque, limonite-bearing layered chalcedony with iridescence
which is created through diffraction of the light by the
layered structure."
For some reason, I seem to remember seeing somewheres that the
irridescent “globes” one sees are referred to as “oolites” or
“oolitic” something or others…
Curiously, in a book with lots of pictures, Schumann’s did not
picture any fire agate…
Good place to see just how amazing this stone can be…
The ring I wear the most is a basic fire agate (courtesy Gerry
Galarneau), brown background that puts out an interesting variety of
colors depending upon the light…
It’s a durable stone, too…I’m hard on everyday jewelry…or
maybe just a klutz……
The fire in fire agate is only found in chalcedony which is colored
a characteristic brown by limonite, an iron oxide. The fire does
only occur in thin layers. Fire agate does frequently delaminate,
and when it does you can see that the color is an irridescent
surface on a layer of chalcedony, it is not within the chalcedony
itself. If this surface irridescence is not irridescent limonite,
what exactly does your friend believe it to be?
In response to your query I would suggest that you initiate a search
usng turgite and goethite. Turgite was commonly available as a rare
Mexican mineral during the sixties and seventies. It was an
irridescent botryoidal rock that had very flashy colors. Turgite has
been discredited as a sepearte mineral species because it is a
combination of goethite and hematite.On the other hand I have
observed in the field some fantastic examples of what might have
been turgite at the Royston Turquoise mines near Tonopah , Nv. The
probability that this might have been the basis for the irridescent
coloration of fire agate is inescapable. Happy querying…
The globes could be referred to as “oolitic,” I can see the basis
for the reference, but oolite is petrified fish eggs, and this could
lead us to a whole other area of confusion between stones…
The irridescent globes are more commonly commonly referred to as
botryoidal (resembling a bunch of grapes.)
I have to differ with you on what oolite is. While doing my geology
degree, I was taught that oolite is a synonym for oolitic limestone,
which forms in sedimentary beds from ooliths (from the Greek for
egg). An oolith is defined as a spherical or subspherical rock
particle which has grown by accretion around a nucleus, such as a
grain of sand or a shell fragment. The majority are calcareous, but
may occur in other forms, such as oolitic ironstone which is a
sedimentary iron ore. Ooliths are tiny, and larger ones (3mm and up)
are called pisoliths, beds of which are called pisolite.
There is an uncommon type of oolith which consists of a thin skin of
calcite deposited over a perfectly spherical quartz grain. However,
visually a piece of oolite does resemble fish roe, so perhaps this is
where the idea of petrified fish eggs came from.
‘Lower Oolite’is an alternative term for the Middle Jurassic, and
’Middle and Upper Oolite’ likewise refers to the Upper Jurassic.
These are the periods during which most oolitic limestone was laid
down in Britain.
I hope you weren’t serious when you said that oolitic meant
petrified fish eggs. Oolitic does refer to formations that look as
if they might be like fish eggs, but that is about as far as it
goes. Most oolitic forms are in reference to round forms in a matrix
of contrasting material. Ocean jasper is a good example as is Poppy
Jasper from here in California. Limestone is another form that is
sometimes oolitic. Fish eggs are never contained in a calcarous
shell that I know of, therefore the likelihood of their being
pertrifiied is slim to none.Microscopic examination of oolites
usually reveals a concretionary structure formed around a nucleus.
Sometimes the oolite will have microspic quartz crystals radiating
outward.
The forms that are usually asociated with fire agate can probably
best be described as botryoidal which means grape like as in a bunch
of grapes. Sorry to be a nitpicker !
I seem to have missed the original note on fire agate. From the
comments below, I wonder if the confusion has to do with mistaking
the words “fire agate” and “fire opal”. The fire in opal is the
result of a reflection of light from a grid of spherulitic (is that
the right word?) particles…little round particles arranged in a very
orderly fashion.
On the other hand, oolite refers to a sedimentary rock made up of
round particles of mineral that precipitate from water. Oolite means
like fish eggs. I think it refers primarily to the size of the
precipitates.
Opaque, limonite-bearing layered chalcedony with
iridescence which is created through diffraction of the light by
the layered structure.
For some reason, I seem to remember seeing somewheres that
the irridescent "globes" one sees are referred to as "oolites" or
"oolitic" something or others..
Ron, I wasn’t kidding, but I know now that I was mistaken regarding
oolite being petrified fish eggs.
When I was a kid, and a budding rockhound, an old lapidary showed me
his collection, which included a slab of oolite, which he earnestly
informed me was petrified fish eggs. I didn’t think to question at
the time, and never did since until this thread. Now I know that the
term oolite refers largely to sedimentary masses of spherules which
resemble fish eggs. It was a common misunderstanding, I think- see
here-
Turitella agate was thought to contain petrified fish eggs, way back
when.
As far as Ocean Jasper and Morgan Hill Poppy Jasper, I am familiar
with the materials, but have always heard/read them referred to as
“orbicular” rather than “oolitic.”
At any rate, fire agate with the bubbly surface is properly
botryoidal, as in botryoidal chalcedony, not oolitic, as we would
both agree that it is not a concretion of fish-egg like spherules.
An oolith is defined as a spherical or subspherical rock particle
which has grown by accretion around a nucleus, such as a grain of
sand or a shell fragment...
What about ocean jasper? Would its orbs qualify as oolitic too?
Thanks and this an interesting thread!
The American Heritage Dictionary gives the definition of “oolite”
as:
A small, round, calcareous grain found, for example, in
limestones.
Rock, usually limestone, composed of oolites.
The Columbia Electronic Encyclopedia has this to say about “oolite”:
Rock composed of small concretions, usually of calcium carbonate,
containing a nucleus and clearly defined concentric shells. In the
British Isles, oolitic limestone is characteristic of the middle and
upper Jurassic, which was formerly termed the Oolite on this
account.
So, given all that, I think botryoidal (AHD: “Shaped like a bunch of
grapes. Used especially of mineral formations: ‘botryoidal
hematite’”) was exactly the word you were looking for.
At any rate, fire agate with the bubbly surface is properly
botryoidal, as in botryoidal chalcedony, not oolitic, as we would
both agree that it is not a concretion of fish-egg like spherules.
Ron, Lee and the others that have responded to my post of “oolite”
and “oolitic” in reference to fire agate…
As I said…though I had seen it somewheres…
Will expunge it from my memory instanter……
Botryoidal chalcedony 'twill be forevermore… And when puzzled
folks ask about botryoidal, I shall mention grapes…
I have come across references to the orbicular jasper around San
Francisco Bay as being a fossil. But I have never seen anything that
really addrsses that subject. Can anyone enlighten me as to where
the original came from, or is it just a conjecture?