Placer Diamonds

Introduction

Wyoming attracts thousands of prospectors and rock hounds who search the hills, valleys, and streams for gold, jasper, gemstones, and other attractive rocks and minerals. On any given weekend during the summer, hundreds of people search for gold with metal detectors, gold pans, and dredges. However, many of these treasure hunters may be overlooking some extraordinary treasure.

Kimberlitic indicator minerals include a group of rare and unique minerals that often accompany diamond such as pyrope garnet (a rare purplish variety of garnet), chromian diopside (distinct emerald-green diopside), high-magnesian chromite, and picroilmenite (non-magnetic, magnesian ilmenite). Such minerals have been found in hundreds of streams, valleys and mountains in Wyoming indicating that the possibility of finding diamonds while searching for gold is high.

Diamond Recognition

In order to search for placer (stream-deposited) diamonds, a prospector needs to become familiar with natural diamond. Diamonds are isometric and have high symmetry. In their simplest form, they occur as a cube. One of the more common habits for diamond is an octahedron (8-sided diamond formed by two-pyramids attached at a common base).

Crystal Habits of Diamond

• Cube: A relatively uncommon habit that is often reported in some placers in Brazil. Cubic diamonds often have several pyramidal depressions on the crystal faces.
• Octahedral: 8-sided diamonds and various modifications are more common. Octahedra often have distinct triangular-shaped growths or depressions on the crystal surfaces known as trigons—these are useful in identification.
• Dodecahedral: 12-sided diamonds with rhombic faces, less common.
• Macle: Twin diamond crystals often occur as a flattened triangular shaped diamond.

Physical Properties

Diamonds have conchoidal fracture, are brittle, and will break from a mild strike with a hammer. Even so, they are the hardest of all natural minerals and are assigned a hardness of 10 on Moh's hardness scale. Diamond exhibits a slightly different hardness in different crystallographic directions, which allows for it to be polished with less difficulty in specific directions.

Diamond has greasy to adamantine luster—they often appear as if a thin film of Vaseline coats them. Diamonds are found in a variety of colors including white to colorless and less commonly shades of yellow, red, pink, orange, green, blue, brown and black.

Special Characteristics

Approximately 1/3rd of all gem diamonds will fluoresce blue when placed under ultraviolet light. Diamonds have tremendous thermal conductivity, such that they will feel cold to the lips when touched, since the gem conducts heat away from the lips. This is why diamonds are sometimes referred to as "ice."

Diamonds are hydrophobic (nonwettable) and repel water. Because they are hydrophobic, diamonds attract grease (grease will adhere to the surface of a diamond), providing an efficient method for extracting diamond from waste material.

Prospecting for Placer Diamonds

Diamonds have moderate specific gravity (3.5) and tend to concentrate with black sands in creek and riverbeds. A prospector should be able to pan for diamonds as one would pan for gold, and Wyoming and Colorado provide excellent hunting grounds for placer diamonds.

When found in streams, diamonds may have been liberated from nearby kimberlite, lamproite, or related lamprophyric pipe or dike, or may have come from diamond pipes hundreds of miles away. Because of the extreme hardness of diamond, some diamonds are thought to be able to resist stream abrasion over great distances.

The Colorado-Wyoming State Line District

In the Colorado-Wyoming State Line district south of Laramie, a minimum of 40 diamondiferous kimberlites has been eroded over a period of 300 to 600 million years. Hundreds of thousands (if not millions) of diamonds must have escaped these pipes during erosion.

The largest diamond recovered from the Kelsey Lake mine along the Colorado-Wyoming border weighed 28.3 carats, and a fragment from a broken diamond was projected to have come from an 80 carat stone. So one should be able to find some large diamonds downstream.

Recorded Placer Diamond Finds

• A 6.2-carat diamond was found in Fish Creek during early testing of the Kelsey Lake kimberlites
• Diamonds were recovered on Rabbit Creek adjacent to the Sloan 1 and 2 kimberlites by a prospector searching for gold
• A prospector recently panned a diamond from the Poudre River using only a gold pan