By Dan May
On Our Land
Dan May
As the gift-giving season approaches, ideas for some turn to jewelry, including precious gemstones like diamonds and rubies or sapphires. Diamonds are the most familiar, and the rating system used to define their value (and that of all gems) is well known, including carat, cut, clarity and color. Red is considered especially romantic.
Carat is the weight of a gemstone, and one carat is one fifth of a gram. Diamonds are nearly pure crystalline carbon, and the crystal structure of a diamond allows it to be cut into the widest variety of faceted shapes among gems. Microscopic inclusions entrained in the 3-D structure linking carbon atoms produce a cloudy appearance and thus lower clarity. Most “clear” diamonds appear colorless, but often have yellow or blue tints. These colors arise when atoms similar in size to carbon substitute for some of that element in the crystalline structure. Nitrogen atoms impart yellow to orange color, and boron yields blue tints.
The rarest of all diamonds are pink to red in color. These hues do not come from element substitutions but arise when the crystalline structure has been distorted by shear pressure. Red diamonds are extremely rare, and only a few dozen have ever been recovered. Most are smaller than one carat but extraordinarily expensive. Larger stones sell for nearly $1 million per carat.
When considering a red gem, rubies most often come to mind. Rubies are composed of crystalline aluminum oxide. The common mineral occurrence for this compound is corundum, and gem quality varieties include both ruby and sapphire. When chromium has replaced some of the aluminum in the crystal structure, the distinctive ruby red color results. If iron and/or titanium replaced some aluminum, sapphire blue and other colors appear. High quality rubies sell for more than $10,000 per carat.
If you desire to purchase a red gem, but even rubies are too pricey, garnets can be an attractive semi-precious alternative. You might even get lucky and find one in the rocks around Milford or Orange. The dark red garnet mineral known as almandine is the state mineral of Connecticut and is often found in the bedrock that underlies this area, especially near Eisenhower Park.
When hiking there, if you see a small dark pebble-like mineral sticking out of a smooth rock face, and it looks somewhat like a stop sign in cross-section, it is apt to be an almandine crystal. The most common shape for a well-formed garnet is a 12-sided crystal with diamond-shaped faces. It is the model for the 12-sided dice used in Dungeons and Dragons. Most are only a few millimeters in size, but some approach a centimeter in width.
Garnets in Milford and Orange are usually found in thinly layered dark schistose rocks that are interspersed with coarser, lighter gray gneissic rock. The schistose layers are more easily weathered and eroded than gneiss and often underlie local stream and river valleys. If you notice a backhoe excavating bedrock near a stream, you might find nice garnets in excavated debris.
However, the easiest place to find almandine is at a nearby hardware store. Garnets which are not jewelry-grade are crushed, sorted to various sizes and glued to card stock to make the premium reddish sandpapers used to smooth and finish wood. The smaller grit numbers (40 to 200) are progressively finer-grained sand sizes, and higher numbers are even finer silts and powders. Number 800 and higher red sandpapers are basically garnet dust.
All gems are known for their hardness. Mineral hardness is the capacity of one mineral to scratch or abrade another. On a 10-point scale, talc is the softest mineral with a hardness of 1 and diamonds are the hardest of all at 10. Most metals are below 7, and woods are almost all less than 3.
Garnets have a hardness of about 8, and corundum (and ruby) has a hardness of 9. Corundum is also a common abrasive used for sandpaper and polishing wheels for metal and wood. If you use gray-colored sandpaper, you are likely using crushed corundum. Professional manicurists use both red (garnet) and gray (corundum) emery boards to file nails.
Diamonds are rarely gem-quality and typically are crushed and embedded as abrasives in rock cutting and drilling tools. They cut all other solids. If you cannot afford a multi-carat gemstone, you might consider buying your loved one some sandpaper, emery boards, or diamond-encrusted masonry saw blades, along with a stereomicroscope to view these very small but amazing crystal fragments. They look spectacular when magnified 100 times.
Dan May is a local geologist.