The random letter was L this month.  I chose a very attractive L mineral and one that catches the eye of everyone that sees it.  Many people have nice examples of labradorite in jewelry or your rock collection somewhere so please post up your examples if possible.  Thanks!

Labradorite

Labradorite ((Ca, Na)(Al, Si)4O8), a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573. Twinning is common. As with all plagioclase members, the crystal system is triclinic, and three directions of cleavage are present, two of which form nearly right angle prisms. It occurs as clear, white to gray, blocky to lath shaped grains in common mafic igneous rocks such as basalt and gabbro, as well as in anorthosites.

Occurrence

The geological type area for labradorite is Paul's Island near the town of Nain in Labrador, Canada. It has also been reported in Norway and various other locations worldwide.
Labradorite occurs in mafic igneous rocks and is the feldspar variety most common in basalt and gabbro. The uncommon anorthosite bodies are composed almost entirely of labradorite. It also is found in metamorphic amphibolites and as a detrital component of some sediments. Common mineral associates in igneous rocks include olivine, pyroxenes, amphiboles and magnetite.

Labradorescence

Labradorite can display an iridescent optical effect (or schiller) known as labradorescence. The term labradoresence was coined by Ove Balthasar Bøggild, who defined it (labradorization) as follows:
Labradorization is the peculiar reflection of the light from submicroscopical planes orientated in one direction (rarely in two directions); these planes have never such a position that they can be expressed by simple indices, and they are not directly visible under the microscope.
Contributions to the understanding of the origin and cause of the effect were made by Rayleigh (1923), and by Bøggild (1924).
The cause of this optical phenomenon is phase exsolution lamellar structure, occurring in the Bøggild miscibility gap. The effect is visible when the lamellar separation is between 128 to 252 nanometres (5.0×10−6 to 9.9×10−6 in); the lamellae are not necessarily parallel; and the lamellar structure is found to lack long range order.
Relationships between lamellar separation and chemical composition have been found for the An (Anorthite) content (calcium rich plagioclase) of the mineral.
Some gemstone varieties of labradorite exhibiting a high degree of labradorescence are called spectrolite.

The Wikipedia reference page:  https://en.wikipedia.org/wiki/Labradorite

I purchased this from a Gem and Mineral show last year in San Jose, Ca. The way this piece lit up was hypnotizing and I couldn't help myself. Had to get it.

magical! 

There are many locations in the area around Mt. Marcy and a quarry near Blue Ridge NY.  Labradorite color changes from differing angles and is called the schiller effect. I built a fireplace  with it. You can also buy tile or counter tops made of it. They call it Blue Pearl.

https://www.facebook.com/AmazingGeologist/photos/a.398226493604029.92268.3982228...For its size, this is the finest Labradorite I have ever seen.               

Quick labradorite question if anyone has any thoughts on this:

I've found that when I put a bunch of mixed material in the rock tumbler, any labradorite in the batch usually seems to fail to polish up, even after all four stages / grit levels. Yet when there's both labradorite and moonstone among the material, the moonstone comes out fine. Since they're both feldspars and theoretically the same hardness, is there something else about labradorite that causes it to tumble so poorly? Is this something others have seen happen, or is it possibly something I'm doing wrong? Do I need to accumulate enough labradorite to tumble a batch of it on its own?

Thanks for any advice!

Doug

That makes sense. Thanks, Joe.