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.