Sunday, April 5, 2020

The Uncommon Beauty of Common Opal

Pendant of Precious Opal






Before I became a rockhound, I thought that opal was the flashy stone often found in rings and earrings.That type is called precious opal.  I love it, but as a lapidary I find it terribly expensive and that makes me nervous to work on it.










For lapidary purposes, I personally prefer common opal. It comes in an amazing range of colors (Colorless, white, yellow, red, orange, green, brown, black,  and blue) and patterns, making it a great material for my lapidary fun. I've even used it in two of my lapidary meal displays!

The Opal Fish slab shown with veggies



The first part was a hunk of white, banded common opal that looked for all the world to me like a slab of cooked halibut. That led me to create my first meal case I show at lapidary club shows and a county fair. Notice how the natural lines in the opal help give it that "fishy" look!






Green Opal Pickle Relish

The second was a chunk of green opal that made me think of hamburger pickle slices. I'd just started working on a hamburger meal at the time and I blythely put it into a saw to slice without examining it closely. This was the moment I learned about how easily common opal
can fall apart. The sound it made when it exploded in the closed saw was amazing.  Stopping the saw and opening the top, I was treated to a view of thousands of small bits of green opal everywhere in the saw, including the oil. Once it was cleaned up, I decided the bits looked a little like pickle relish. So I added a few small shards of red glass, and my repurposed opal was ready.

OTHER THAN AS A FOOD SUBSTITUTE

Australian Dendritic Opal



The range of common opal that is suitable for cabochons is almost limitless. The solid colors are beautiful, but since opal tends to take more of a lustre than a shine, opal with dendrites and other inclusions makes a cab with a story to tell.

In my shop, I like to work with this material. At between 5.5 and 6.5 on the Mohs Scale, it works fairly quickly, but is hard enough to hold up a bit and not disappear under the wheel.  Like I said above, the finishes aren't shiny, more of a luster. The dendritic  "kite" shows a shine at the center because the light was right over it there.

Here's a lovely piece of Blue Opal I'm looking forward to working on. It has a few gold-colored imperfections, but to me,they help tell the story.


SHOP TALK

Because common opal is amorphous (lacking a natural shape, not crystalline: perhaps a geletain
mineral), it is classed as a mineraloid. It also contains water that can range from 3 to 21 percent of its weight. It occurs in fissures of other rocks, giving it its shapes. That moisture level is why some dry out and crack. If you've purchased some opal at a show and it is in a container with water, it's probably best to keep it that way or it may craze, fracture, or lose its color.

Boulder Opal from Australia








I chose this boulder opal, not because of the precious flashes--it doesn't have any--but because the clarity of the opal shows how the bits of debris settled at the bottom when the opal was still a gelatin. If you can enlarge this image on your screen, you'll see what I mean.






 SOMETIMES - JUST LOOK AT IT




Especially when I'm at shows, I tend to find opal specimens that are just incredible to look at, and I'd never want to cab.  This limb-cast is from the Blue Forest in Eden Valley, WY--which was a forest once, eons ago, but is desolate now.


Blue Forest Opal

Please send your comments about common opal to Donna@LapidaryWhisperer.com

Be sure to visit my next blog post on May 5. You'll love it!

Donna
Your Lapidary Whisperer
  






Thursday, March 5, 2020

DO YOU DETECT METAL IN YOUR CABOCHONS?

Cabochon with Azurite
You know how some of the minerals we use to create cabochons contain metals as an integral part of their makeup, like Azurite Cu₂, and Malachite Cu₂CO₃ (OH)₂.in which copper is part of their chemical makeup. Yet there are times when a metal is side-by-side with a mineral in a slab and that situation allows you to make some unique cabs.
 LAPIDARY NOTES

My experiences in working with materials that contain metal is all over the board.  For example, the Azurite above has a hardness Mohs rating of 3.4 to 5 and Copper comes in at 3. However, if the material is in matrix, the harness of the matrix must also be taken into consideration.  With the mixes pictured below including the copper and epidote are wildly different with the epidote coming in at 6-7 on the hardness scale making it very difficult to avoid undercutting and creating a very difficult situation when it comes to polishing.
A LITTLE BACKGROUND
Scientifically, when you get a slab that has metals and minerals, here's what you're dealing with:

METAL- (Naturally occurring) consists of a single atomic element, like iron, copper, etc.
MINERAL - a combination of several elements and is generally crystalline, some of which can be processed into metal.
Here are a few mineral/metal combinations (and not-exactly rocks) you'll probably deal with in your cab shop:


COPPER -  A Metal
For example, check out this nifty copper cabochon with an epidote matrix (according to the vendor I purchased it from) has the copper peeking through it.
Copper and Epidote
 I've heard that this kind of specimens can sometimes have quartz in it, which would have lightened its look a bit, but all in all, I like the blue-green

TIGER IRON - Mineral & Iron Oxide
However, this Tiger Iron cab? Not exactly. 
Tiger Iron


The first time I saw a slab of Tiger Iron, I thought it was a very poor quality of Tiger Eye. But I was wrong.

Tiger Iron is a separate kind of mineral that combines Tiger Eye and Hematite in random patterns. Hematite itself, is a form of Iron Oxide which is where the "Iron" in Tiger Iron comes from. Depending on the amount of Hematite vs Tiger Eye, it can be very attractive. However, I prefer a lot more Tiger Eye than Hematite, so this isn't a favorite among my cabochons. The dark overwhelms the chatoyance. I see this from time to time at rock shows, but if I'm doing the buying, I'll get straight Tiger Eye any day.


PYRITE  - Mineral

Pyrite in Quartz
I admit I love this cab!  It's quartz with pyrite  embedded in it. Because of the orange in the quartz and the shapes of the markings, it looks a lot to me like a magical fire. I wasn't able to catch the flash from the pyrite in a picture, but it's very impressive in person.

The Pyrite is a mineral (Iron Sulfide) and it's also known as "Fool's Gold". In some settings, like this, it actually looks more like the Iron it contains than gold, but if you've seen pyrite crystals, you understand where the name originated.


Tin Oxide in Quartz



RUTILE - A Mineral

 When I first started admiring Rutilated Quartz, I thought that the different colors of the thin lines meant that they were different materials. Not so. According to Mindat.org, Rutile (TiO2) comes in Blood red, brownish yellow, brown-red, yellow, greyish-black, black, brown, bluish or violet.

Oxides are actually found in many stones. In rutilated quartz, the rutiles  can be made of tin oxide.   That's the same stuff in the white paste lifeguards at the beach smear on their noses to prevent sunburn. More importantly, in its dry form, it makes a wonderful lapidary polish.





Do you have any inclusions that you want to identify as either metal or mineral? Send me the information and a picture and I'll see what I can do to help you identify it.


Until next time,

Your Lapidary Whisperer,

Donna