In the scientific world, “Luster” is a characteristic of minerals that refers to how the surface reflects light. It is the surface reflection of light that defines the luster of a mineral. There are many different terms for luster, including Chatoyance, Luster, Streak, and Play of Colors. Learn more about mineral luster and how to find a mineral with the desired lustre.

Luster

The luster of a mineral refers to its appearance when reflected light hits it. It may be metallic, glassy, or dull. Depending on its chemical makeup, a mineral may have multiple levels of luster. In rock and soil, lustre refers to the way a mineral reflects light. It is determined by the intensity of specific colors, and can range from matte to highly reflective.

A mineral’s luster is an important property in distinguishing between metallic and nonmetallic minerals. For instance, a dark magnetite sample can be mistaken for a ruddy, earthy hematite if it lacks luster. Although the two are similar in appearance, they differ in the magnetism of the former. In contrast, non-metallic minerals have a low luster, and lack of lustre is a sign of poor conductivity of electricity and heat.

Chatoyance

Some minerals have properties that make them resemble “cat’s eyes.” Such inclusions are usually parallel and are known as chatoyant. The light that strikes them is reflected at right angles to the length of the inclusion. This property is typical of bronzite and satin spar, which are both mineral varieties. Other minerals that exhibit chatoyance include diopside, garnet, peridot, and quartz.

Crystals with a strong chatoyance effect look as if they are made of two materials. A cat’s eye, for example, appears to separate the stone into light and dark-colored material on one side. This effect is called the “milk-and-honey effect.” The photo above shows a cat’s eye chrysoberyl in action.

Streak

The way a mineral reflects light depends on its crystal structure. In order to identify a mineral by its streak, a finely powdered sample is used. The streak is more useful than the entire sample. A streak plate is made by rubbing the mineral powder onto the unglazed back of a white porcelain tile. Some minerals do not streak. For this reason, you should not use the whole mineral sample for identification.

Luster is the shiny appearance of a polished metal. Minerals with metallic luster include pyrite, chalcopyrite, bornite, and magnetite. Pyrite is a mineral that is well-crystallized and contains a lot of iron, and other metallic minerals include hematite (Fe2O3) and stibnite (Sb2S3). Most metallic luster minerals are sulfides or oxides, which are native elements.

Play of colors

One of the most striking and beautiful effects of gemstones is the Play of Colors, or “opalescence,” which refers to the shimmering iridescence caused by the reflection of light off the mineral. Opals can have a patchwork-like appearance that progressively dissipates as you move them. This effect is similar to the pearly luster taken to its nth degree.

The color of minerals is due to several factors, including the wavelengths of light that they absorb and reflect. Chlorophyll, for example, absorbs red and blue light and reflects green and yellow wavelengths. The atomic structure and chemical impurities also affect the colors of minerals. It’s important to note that these color differences are only observable in a small number of minerals, but they’re helpful nonetheless.

Cleavage

Crystals show cleavage, but the way they separate is not always easy to distinguish. Some minerals exhibit a perfect cleavage while others have poorly developed cleavage. Cleavage is a feature of many minerals, but it is a key characteristic that can be used to identify specific minerals. Below is a look at the difference between crystal faces and cleavage surfaces.

Crystals are often fractured, which means they break along planes, or planes. In some cases, this cleavage is visible after the mineral is extracted from its rock. In other cases, a mineral may break along planes, which will be visible when it is cut or drilled. This is called a “perfect cleavage.”