Diamonds

This post is the second part of a "two-parter." See my previous post on graphite, which is an allotrope of diamond. An allotrope is each of two or more different physical forms in which an element can exist. Graphite, diamond, and charcoal are all allotropes of carbon. 

Like graphite, diamond is a native mineral. That means it consists of one element, which is carbon, with a chemical formula of C. Diamonds are measured in terms of carats, which refers to a diamond's weight, not its size. One carat = 200 milligrams = 0.200 grams = 1/5 of a gram. The "largest" diamond known is the "Golden Jubilee" (5,000 grams).

Most  diamonds are Precambrian in age (between 3.5 to 1 billion years old). They are formed at high temperature and pressure deep in the Earth's mantle, mostly between 150 to 250 km (93 to 155 miles) depth. Some however come from as deep as 500 miles. They can be carried to the Earth's surface by volcanic eruptions and occur mostly in the plugged-up necks of ancient volcanoes, in rocks known as kimberlites. If the kimberlites become exposed at the surface of the Earth, they become eroded and the diamonds can be concentrated in stream or beach deposits. One of most famous diamond localities is from South Africa, where diamonds have been weathered out from kimberlites and concentrated as placer or "lag material" in beaches.

Diamonds (very small size) have also been found in some meteorites. 

Diamond is the hardest naturally occurring substance, having a rating of 10 on the Mohs Hardness Scale (0 to 10). The arrangement of  the atoms in diamonds is classified under the isometric crystal system, and the resulting structure is extremely rigid. Many diamonds are pure, and as a result they are transparent and colorless. Others can have color, caused usually by the presence of minute impurities. The range of colors (with increasing rarity), along with the known impurity or other factors causing the color, are as follows: yellow (nitrogen), brown (defects), blue (boron), green (radiation exposure), black (referred to as "Carbonado"), gray, pink, orange, purple, and red (most rare of all diamonds).

A diamond, or a simulant? It is 3.5 mm in diameter.

The image above shows a round, "brilliant cut" of what is probably a simulant (synthetic) and not diamond. I found it in a parking lot, and I spotted it about 20 feet away from me, because of its extremely bright and very eye-catching display of reflected light. Part of its mounting was still attached and consisted of hardened copper.

I spent considerable time and effort in trying to determine if what I found is truly a diamond, or if it is a simulant (synthetic or "fake"). 
I tried all the "easy" tests [e.g., water, fog, scratch, newsprint], all of which have been shown as videos on the internet, to determine what I found, but the tests were not conclusive for my mystery "stone." 

There are several types of simulants (note: they are much less expensive that diamonds). Two of the most common ones are moissanite (silicon carbide) and cubic zirconia (zirconium dioxide). 

Moissanite is a very rare naturally occurring mineral, but most of it that is used to make jewelry is created synthetically in a lab. It has a hardness of 9.5, colorless and has more brilliance (flash-of-light, or "fire") than diamond. It is one of the best substitutes for diamond.

Cubic zirconia is an entirely lab-created substance. It has a hardness of 8 to 8.5, and it can scratch easily.  

Based on the fantastic brilliance in natural light of my mystery stone, as well as on the copper mount, I believed that it is probably moissanite and not a diamond. As I viewed numerous websites about diamonds and simulants, I discovered that some simulants can look really good. I discovered eventually that the best and most definitive way to distinguish between a cut diamond and the various simulants is to consult a gemologist or a reputable jeweler. Using a very precise machine, they will weight the material in question and compare it with a comparable-sized diamond. This kind determination will be reliable.

Pyrite cubes

Pyrite is a mineral that most people have either heard about or seen. It superficially resembles gold, yet the chemical and physical properties of pyrite make it easy to distinguish it from gold. The main differences are listed below:

Pyrite is an iron sulfide, with the chemical formula FeS2. Gold's chemical formula is simply Au.

Pyrite crystal system is isometric (cubic), and crystals formed under perfect conditions will be cubes (as shown below). Gold is rarely found as crystals; rather, it occurs in nuggets, irregular blobs, or small flakes. It cannot occur in cubes.

Single cube of pyrite, width 2.9 cm (from Spain).

Cluster of intergrown pyrite cubes, total width 5 cm long (from Spain).

Pyrite is harder with a value of 6.5 on the Moh's Hardness Scale [i.e., a scale with talc and graphite the softest minerals (value of 1), and with diamond the hardest mineral (value of 10). Gold has a value of 2.5. Gold is very soft; so much so that other elements (e.g., copper, nickel, or platinum) have to be added to it (in the form of an alloy) in order to make jewelry.

Note: An ordinary steel knife (hardness value of 4.5) cannot scratch pyrite but can easily scar gold. The superior hardness and brittleness of pyrite also cause it to smash into bits if struck with the tip of a high-quality knife or shatter into small pieces, if hit with a hammer.

Pyrite's streak (its powdered from when scratched across an unglazed porcelain plate, called a streak plate), is black. Gold's streak is brassy yellow.

Small irregular piece of pyrite with its characteristic black streak on a "streak plate.

Pyrite is less dense, and small flakes normally wash away when placed under running water. Gold flakes are very dense and will sink. This is why "gold panning" works so well for finding gold.