Gemology Hub | Gemstone & Mineral Identification Guides

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A beach can look pale one day and show dark streaks after the waves have moved the sand. These bands may contain a higher proportion of heavy mineral grains. Sand is a mixture Sand describes grain size, not one particular substance. A sample can contain several minerals that came from rocks worn down elsewhere. Rivers, wind, waves, tides, and coastal currents move and rework those grains. The grains do not all travel in the same way. Nature does the sorting Density, size, and shape affect how sediment is transported and deposited. Repeated movement can concentrate denser minerals into patches or thin bands. US Geological Survey studies of the Atlantic Coastal Plain describe heavy mineral sands containing minerals such as ilmenite, rutile, and zircon. Some deposits also contain minerals that carry rare earth elements. Does dark sand mean valuable gems? No. Colour alone cannot iden...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A pink or pale feldspar crystal may contain thin, lighter streaks. Some of these patterns are perthite: an intergrowth of two feldspars within what appears to be one grain. A mixture that separates while cooling At high temperatures, alkali feldspar can contain both sodium and potassium in its structure. As conditions change during cooling, an intermediate composition can separate into sodium-rich and potassium-rich parts. Geologists call this process exsolution. The separation happens within the solid material. It does not require the crystal to melt again. What the streaks contain In perthite, the main host is potassium-rich feldspar. The lighter intergrown material is commonly albite, a sodium-rich feldspar. The intergrowth may appear as thin lines, patches, or irregular bands. Some examples are visible by eye. Finer examples need a microscope, and very fine intergrowths requi...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. Some crystals look as though their centres have been scooped out. Instead of a flat face, they show a rim and a series of steps leading inward. This can be a growth form called a hopper crystal. The edges grow ahead In hopper growth, the edges advance faster than the centres of the faces. The result can be a recessed or stepped face rather than a completely filled surface. Mindat defines a hopper crystal by this difference between edge growth and face-centre growth. The term describes the shape; it is not the name of a separate mineral. Salt offers a clear example Researchers studying sodium chloride crystals have produced hopper forms from strongly supersaturated solutions. In simple terms, the solution contains a large amount of dissolved salt compared with what it can comfortably keep dissolved under those conditions. The experiments show that growth rate and the supply of mat...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A crystal's shape is a useful clue, but it does not always reveal the material now inside it. Sometimes one mineral changes or is replaced while its old outer form survives. The result is called a pseudomorph. An old shape with a new identity The word means false form. A pseudomorph can preserve the appearance of an earlier mineral even though its composition or internal structure has changed. Replacement can happen gradually. Other processes involve material coating an older crystal or filling a space left behind. The details matter when describing a particular specimen. A museum example The Australian Museum holds flower-like specimens of chrysocolla replacing azurite from the Dorothy Mine in Queensland. Their striking forms record a mineral change, rather than simply the normal growth shape of chrysocolla. What does “after” mean on a label? A label such as “chrysocolla aft...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A round stone can be tempting to open. Before expecting a sparkling centre, consider another possibility: it may be a concretion. What is a concretion? A concretion forms when dissolved minerals cement sediment together in a particular area. Cement can build around a small starting point, making a harder mass within softer surrounding rock. As the surrounding material wears away, the harder object may stand out or come loose. Some concretions look like balls, while others have less regular shapes. How is a geode different? A geode develops around a cavity, often with minerals lining the inside. The Dugway geodes in Utah, for example, formed in cavities in volcanic rock. Mineral-bearing water later deposited material inside them. A concretion is generally a cemented mass rather than a crystal-lined hollow. The two objects can look similar from outside, so roundness alone does not ...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A wall of tall stone columns can look as though someone built it. At places such as Devils Postpile in California, the pattern formed naturally as lava cooled. Cooling creates cracks Basalt is a volcanic rock. As a lava flow loses heat and contracts, cracks can divide it into columns. Geologists call these fractures columnar joints. The cracks develop in relation to the cooling surfaces. This is why many exposed columns look upright, while other examples can lean or curve. Why so many have six sides Hexagonal columns are common, but the pattern is not perfectly uniform. Columns can have different numbers of sides. At Devils Postpile, the US Geological Survey reports that about 55 percent have six sides. That variation is useful to remember. Nature can produce an orderly pattern without making every part identical. Columns are not single crystals A large basalt column is a piece o...