crystals

Organized, homogeneous minerals. Rock collectors love to find crystals.

Chemical Composition of Gemstones

Here’s a neat infographic from Compound Interest (one of my favorite websites) that describes 16 different gemstones and why they have different colors. It also includes their chemical formulas and hardness on the Mohs scale.

Many gemstones would be colorless or a different color if not for the presence of small amounts of transition metals such as chromium or titanium. For example, you can see that aquamarine and emerald both have the same chemical formula Be3Al2(SiO3)6, but emeralds are green because of chromium ions replacing some of the aluminum ions and aquamarines are blue because of iron 2+ or 3+ ions replacing some of the aluminum ions. Click through to read the whole article, because there are many other ways that gems and minerals get their colors!

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Sapphires in the Rough

16 small irregularly shaped translucent stones ranging from colorless to blue.

Frederick, one of our members who is into gem cutting, brought in these rough sapphires from Montana at our last meeting. Check out his website http://missouririversapphire.co/ to see how beautiful they look when they are cut.

Scepter Quartz

Article by Amir Chossrow Akhavan, http://www.quartzpage.de/gro_text.html

A scepter quartz is often defined as a quartz crystal that has a second generation crystal tip sitting on top of an older first generation crystal. The second generation tip typically becomes larger than the first generation tip, but might also become smaller. A scepter can be shifted sideways and does not need to be centered on the first generation tip.
However, there is a problem with a definition that is based on the idea of a second generation: scepters do not only occur as a second generation on an older crystal, they also form stacks of parallel grown crystals that developed at the same time, very often as skeleton quartz. Another difficult case are reverse scepters in which the scepter is smaller than the underlying tip. Here the smaller tip very often does not show any properties that clearly distinguish it from the rest of the crystal and that would justify calling it a second generation. Instead, the crystals often appear to have grown continuously into the reverse scepter or multiple scepter shape.

In all cases, the scepter develops from the already present crystal lattice of the crystal underneath. Thus, to be a scepter quartz, the “second generation” crystal’s a- and c-axes need to be oriented parallel to the respective axes of the “first generation” crystal; just one crystal on top of another doesn’t make it a scepter. Such a crystallographically well defined intergrowth of different minerals is called an epitaxy. In a sense, a scepter represents an epitaxy of quartz on quartz, and because it is the same mineral, it is sometimes called an autotaxy.

Scepters are quite common in certain geological environments. Amethyst from alpine-type fissures in igneous and highly metamorphosed rocks usually occurs as scepters on top of colorless or smoky crystals (not only in the Alps, but for example also in southern Norway or northern Greece). Here, the amethyst generation grew at lower temperatures than the first generation quartz. The same growth form can be observed in pegmatites and miaroles in igneous rocks (for example, amethyst scepters from the Brandberg, Namibia, or from pegmatites in Minas Gerais, Brazil).

Scepters, or to be precise, the “second generation” part of a scepter quartz that defines it, commonly have a number of morphological properties:

  • Scepters are commonly of normal habit and are never tapered. The underlying “first generation” crystal may show a Tessin habit, but the scepter on it will not.
  • Scepters tend to assume a short prismatic habit. An apparent exception are reverse scepters and the normal scepters associated with them, which may occur as elongated extensions of a “first generation” crystal, but then in the shape of multiple stacked scepters.
  • Many scepters show only a weak striation on their prism faces, sometimes it is even missing.
  • Scepters do not show split growth patterns.
  • Scepters rarely show trigonal habits with very small or missing z-faces. An exception are reverse scepters and the normal scepters associated with them.
  • Scepters are often associated with skeleton growth forms (skeleton or window quartz).
  • Scepters commonly show a color, color distribution, diapheny and surface pattern that is markedly different from the underlying “first generation” crystal. Often they are more colorful and transparent. Amethyst scepters are very common, smoky quartz scepters -often with uneven color distribution- are common. An exception are reverse scepters and the normal scepters associated with them which seem to either not differ from the “first generation” or show gradual transitions.
  • Summarizing the exceptions above: Reverse scepters and the normal scepters associated with them seem to have a different set of properties.

Formation

One theory is that a scepter forms when crystal growth is interrupted and parts of the crystal are covered with some material that inhibits further growth. The growth inhibiting material might be only present as a very thin layer and invisible. The very tip of the crystal or the entire rhombohedral faces remain free of that material, and should the conditions change again, the crystal continues to grow from the tip.

One of the problems with that theory is that you would expect to see a larger number of “double”, “triple” or “quadruple scepters”, specimen in which the growth had been interrupted several times and in which scepters with slowly changing habits are stacked. In nature, however, you see a strong dominance of “simple” scepters that consist of just a prism with “a single head”. If you see multiple scepters, then often alongside simple scepters, although multiple changes in the environment should have affected the morphology of all of them equally.

Another problem is that you would not expect to see a fully-grown scepter that encloses the former tip like an onion if the crystal simply started growing from a single point on the surface of the tip. Such a crystal would finally grow into an elongated crystal and would at best assume the shape of a reverse scepter.

As I’ve mentioned, amethyst from igneous and metamorphic rock locations all over the world predominantly occurs as scepters. Even if you just take Alpine locations, it is hard to imagine that the environmental conditions in all those locations have undergone a single sudden change that led to a temporary growth inhibition on the crystals, followed by a very distinctive growth pattern, the formation of scepters.

The internal structure of scepters from Alpine-type fissures (and of scepters in general) is perhaps always lamellar, as opposed to the macromosaic structure of many quartz crystals from Alpine-type fissures. Quartz crystals with a macromosaic structure may carry a scepter, but the scepter will then show lamellar structure.

Corundum

Corundum (Al2O3) is a hematite group mineral that has trigonal crystals. It is found all over the world and can be many different colors including blue, red, pink, yellow, gray, and colorless. These corundum crystals are from the Cascade Canyon in San Bernadino, California. They may not look familiar to you, but corundum has some famous relatives. A gem-quality corundum that is red (Cr-bearing) is known as ruby, and a gem-quality corundum that is blue (Fe- and Ti-bearing) is known as sapphire.

Emeralds

Emeralds are the most famous green gemstone. The word emerald is practically synonymous with the color green, and in fact, the name emerald comes from the Greek smaragdos which means “green gem.” Ireland’s nickname “The Emerald Isle” sadly does not refer to any emeralds found there but for the green scenery.

Emeralds are the green variety of the mineral beryl. The famed green color comes from chromium impurities. When beryl appears in other colors due to different impurities it is called aquamarine (blue), morganite (pink), bixbite/red beryl (red), or heliodor (yellow). Emerald rates 7.5-8.0 on the Mohs hardness scale, though it can be brittle. Emeralds are usually found in Colombia (South America) or Zambia (Africa) in granite pegmatites and metamorphosed mica schists. They grow in hexagonal crystals. The most valuable emeralds for gems are transparent rather than opaque, have few inclusions, and are a dark shade of green. Emeralds usually have quite a lot of inclusions, so sometimes people use oil to hide them, but looking at the inclusions can help you tell where the emerald came from. One final fun fact: There is even a faceting method called the emerald cut, which has a rectangular face with 8 sides. It is also known as the octagon cut. The emerald cut works well on emeralds but can be used on any gemstone, even diamonds.

Vanadinite

Lots of flat orange crystals with some small grains of yellow crystals in between.

Photo by Egen Wark

Reposted from our friend Mineralogy on Google+: Vanadinite is a lead chlorovanadate characterized by red to red-orange hexagonal crystals. It is a secondary mineral found in the oxidized zone of lead deposits resulting from the alteration of vanadiferous sulfides and silicates. A member of the apatite group, vanadinite forms a solid-solution series with its phosphate (pyromorphite) and arsenate (mimetite) analogues. It was first discovered in Mexico in the 19th century and is prized by collectors due to its distinctive color.

If you like the photo and writeup, check out Mineralogy’s Google+ page and follow them.

Christmas Rhodochrosite

A cluster of crystals that has square red crystals on a shiny green surface, with white and lavender puffballs scattered across the whole cluster. A nice display of Christmas colors.

Ex. J. Vorpahl Mineral Collection, http://www.themineralgallery.com/rhodoroom.htm

This gorgeous crystal specimen is perfect for Christmas! It has red rhodochrosite crystals on a green matrix (green pyrite?), with pale lavender fluorite cyrstals that look like little snowdrifts. From Steve’s Pocket, Fluorite Raise, Sweet Home Mine, Alma, Colorado.

Snowflakes for the First Day of Winter


We usually feature quartz crystals or fluorite crystals, but today’s crystal is somewhat different. To celebrate the first day of winter, here are some photos of snowflakes taken with a Low Temperature Scanning Electron Microscope (LT-SEM). The photos were taken by members of the Electron and Confocal Microscopy Laboratory, Agricultural Research Service, U. S. Department of Agriculture. From top to bottom, left to right, there are some needle crystals, a snow crystal coated in rime, a hoar crystal, a common snow crystal, a snowdrift from St. Louis Creek, a close-up of packed snow, a side plane crystal, and the traditional dendritic snow crystal. Suddenly I understand why there are so many different types of snow. More photos of snowflakes here.

Antlerite

What is a reindeer’s favorite copper sulfate mineral? Antlerite! Antlerite is named after the Antler Mine in Arizona, but was more often found in Chuquicamata Mine in Chile. That mine has been closed, so antlerite is pretty rare now. This is a close-up of a specimen from the Chuquicamata Mine measuring 7.4 x 3.8 x 2.2 cm.

The whole plate, with fingertips included for scale.