On a much smaller scale, I remember large rock piles on nearly every farm in northern Indiana where farmers tossed large, glacially deposited field stones that interfered with their work. Looking through these rock piles would always yield highly interesting rock specimens. In my hometown, there exists a decorative arched gateway to the city park that was made from field-found stone. Fine examples of granite, porphyry, quartz, and gneiss may be seen jutting from the concrete in which they are embedded. Similar field-stone work can be found in many fireplaces, front porch supports, and home foundations all over the upper Midwest.
Following will be a brief discussion of the materials from which birdstones were made. Also, included (when available) will be photographs of finished birdstones made from these materials and a few examples of the raw material as it may have been found by the prehistoric artists or as fashioned into other types of artifacts.
Two samples of banded slate found in northern Indiana. The sample at left is the more commongray to green banded slate. On the right is the less common, red banded slate. Also shown are two birdstones made from green-gray banded slate. The example on the left is from Clinton County, Michigan. The example on the right is from Allen County, Indiana.
Metamorphic rock such as slate often exhibits wavy or evenly spaced bands or striations from its earlier existence as a sedimentary rock such as sandstone and shale, but it also may be found with apparently no bands or striations. Many different colors of slate may be found. In North America, gray colored slate would predominate. However, green, red, and purple slates are also noted.
What are the reasons for such a high percentage of birdstones being made from slate? First, polished slate is beautiful! The natural beauty of the contrasting bands in banded slate could also often be used to accentuate the final form of the birdstone, and this factor may be far more important than we can ever know today. It is also possible that banded slate was chosen due to its similarity to wood. Perhaps the prior experience of working with the banded structure of wood may have made banded slate a good choice. Second, slate is not as hard as quartz or granite; therefore, the pecking, polishing, and perforating process would be easier than working with the harder materials of porphyry, quartz, and granite. Third, slate may be polished to a very glossy, high sheen.
The term ferruginous slate has been affixed to birdstones having been made from fine-grained, striated or banded material similar to banded slate but often more colorful and certainly having a higher degree of hardness. Some of the most famous birdstones known have been made from what collectors have long termed ferruginous slate. The term ferruginous indicates the material contains iron, giving the material colors tending toward reds and oranges. Indeed these colors may be quite spectacular. Townsend illustrates a few of these rare, exotic beauties in the color plates of Birdstones of the North American Indian. However, this material is more frequently found in much more subtle shades of gray, green, or black that closely resembles banded slate, as shown in the accompanying photograph of the ferruginous slate bannerstone.
This double edged or double bitted axe bannerstone is made from a very fine grained and highly polished hard slate that is commonly referred to as ferruginous slate. This bannerstone was found in Hillsdale County, Michigan.
Porphyry is an igneous rock that was a favored material for birdstone makers. Igneous rock begins life as molten magma beneath the surface of the earth. When magma finds its way to the surface it cools and becomes igneous rock. If the cooling process is quick, the crystals observed in the rock are very small. When the cooling process is slow the crystals formed are much larger. These crystals are commonly termed phenocrysts. Many forms and colors of porphyry are found around the world. Very large phenocrysts set in a contrasting matrix of dark green granite was apparently the type favored by prehistoric birdstone makers. This type of material is shown in the photographs below. Many birdstones made from porphyry do not display such large and showy phenocrysts, but the material is still usually dramatic and quite beautiful.
Many of the more highly developed birdstones are made from porphyry and these are among the most highly sought after by collectors. The material is showy and beautiful, and if not weathered too much over the centuries, the degree of polish can be amazing. A saddle birdstone with popeyes and a fantail would be a great example.
This ntgnty pannarea, targe porpnyry cobble was purposely broken into two portions. It was originally found in Michigan and has a maximum circumference of 19".
This photograph illustrates the interior view of the same porphyry cobble, along with two birdstones fashioned from porphyry similar to the cobble. The birdstone on the left is from Alpena County, Michigan. The birdstone on the right was found in Allen County, Indiana.