![]() So, the real difference between jasper and agate is the amount of impurities and foreign material contained with a specimen. Jasper contains enough impurities and foreign material to render it opaque. When a small amount of impurities or foreign materials are added, the color of the microcrystalline quartz changes, and its ability to transmit light decreases. Microcrystalline quartz in its pure form is semitransparent. Opaque means that neither light nor images pass through. Jasper is an opaque variety of chalcedony. This is why agate nodules are often found in stream valleys that cut through fine-grained igneous rocks or limestone. So when the rocks are eventually broken down by weathering, the durable agates will remain. All of these types of rock are more susceptible to weathering than agate. Crystallization of foreign (non-silica) materials is often what forms the plumes, dendrites, or mossy structures that are often seen in translucent agate.Īlthough agates typically form in igneous rocks such as basalt, rhyolite, and andesite, they can also form in sedimentary rocks such as limestone. These impurities can alter the color of the microcrystalline quartz. If the dissolved mineral composition of the silica-rich water changes over time, impurities (elements other than silicon and oxygen) can be incorporated into the gel and into the microcrystalline quartz. Over time, additional layers of gel are deposited, and these form younger bands of microcrystalline quartz on the walls of the cavity. That gel will slowly crystallize to form microcrystalline quartz. When the solution is highly concentrated with dissolved silica, a silica gel can form on the walls of these cavities. Many agates form in areas of volcanic activity where waters, rich in dissolved silica (SiO 2), flow through fractures and cavities in igneous rocks. These are often translucent agates with plume-shaped, dendritic or mossy inclusions. However, some agates do not have obvious bands. If you hold it up to the light and pass your hand between the material and the source of light, your hand will block the light passing through the material.Īgate is generally a banded material, and observing bands in a specimen of chalcedony is a very good clue that you have an agate. If you hold a translucent piece up to a source of light, you will see a small amount of light passing through the thin edges. ![]() ![]() If you have a piece that is semitransparent, you will be able to hold a very thin piece up to the light and see distorted or foggy images through it. With its colorful appearance, ability to accept a bright polish, durability to survive the tumbler, ready availability, and low price, chalcedony is the perfect material for making tumbled stones.Īgate is a translucent to semitransparent chalcedony. It can also be banded or have plume, dendritic, mottled, mossy or other colorful structures within. It is commonly gray, white, brown, red, yellow, orange and black, but it can occur in any color. Now you know how to tell chalcedony from other types of quartz.Ĭhalcedony occurs in a wide range of colors. Thus, much of the light that strikes chalcedony fracture surfaces is scattered instead of reflected, hence the dull or waxy luster. A disturbance occurs at each crystal boundary, which causes slight irregularities on the curved fracture surfaces. When it fractures, the fracture surface must propagate across thousands of quartz microcrystals. This luster difference is caused by the microcrystalline nature of chalcedony. Some people find that dull to waxy luster surprising because broken pieces of quartz have a bright, vitreous luster. These freshly broken pieces of chalcedony have a dull to waxy luster. Chalcedony has a hardness of approximately 7 on the Mohs hardness scale.Ĭhalcedony breaks with a conchoidal fracture, meaning its broken pieces often have smooth, curved surfaces. That's why ancient people sought out chalcedony for making their tools and weapons (it was often called "flint"). Other types of quartz have much larger crystals - often many inches or feet across.Ĭhalcedony is a very hard and very durable material. "Microcrystalline" means that the quartz is in the form of crystals that are so small that a microscope must be used to visualize them individually. "Quartz" is the name of a mineral composed of silicon and oxygen (SiO 2). Agate and jasper are both varieties of chalcedony. Chalcedony is a generic name given to materials that are composed of microcrystalline quartz.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |