Adularescence(Ad jewel er essence) is an optical phenomena often called the “moonstone” effect. The phenomenon is applied to feldspar, usually orthoclase that, when cut as a cabochon, exhibits a floating, billowy bluish or white light as the stone is turned. Referred to as schiller by mineralogists. Less intense or ‘mild adularescence’ occur more as a general scattering of light which results in a slightly hazy or milky appearance in otherwise non-hazy stones, very frequently seen in common opal and rose quartz.
Asterism (Ass ta riz em) is an optical effect which appears as a star shape within a crystal. The Star-effect or “asterism” is caused by the dense inclusions of tiny fibers of rutile (also known as “silk”). The stars are caused by the light reflecting from needle-like inclusions of rutile aligned perpendicular to the rays of the star. Photograph
Credit: “Asterism” by http://www.gemselect.com
Aventurization (Ah VEN chur rize ation) is the colorful play of glittering reflections of small, leaf like inclusions on an opaque background. The inclusions could be hematite and/or goethite in the case of aventurine feldspar or fuchsite, mica and/or hematite in the case of Aventurine Quartz.
Chatoyant (sha TOY ant) In gemology, chatoyancy or chatoyance also called the cat’s eye effect, is an optical reflectance effect seen in certain gemstones. Coined from the French “oeil de chat,” meaning “cat’s eye,” chatoyancy arises either from the fibrous structure of a material, as in tiger’s eye quartz, or from fibrous inclusions or cavities within the stone, as in cat’s eye chrysoberyl.
Concentric (con SIN trick) Concentric means having the same center, Think of the ripples you see when you throw a rock into a stream. The circles that form out from the center (where the rock landed) are what is known as concentric. True Agates have concentric banding.
Concretion (con CREE shun) A Concretion is a concentration of mineral growth around a nucleus within a sedimentary environment. Mochi Marbles (Moqui Balls) are a great example of Concretion. The outer shell is hematite (iron oxide) and the inner nucleus is sandstone.
Crazing (Cray zing) is a condition that causes internal or surface cracks. Opal, Amber and Garnets can often be seen with this condition. It also can be seen on ceramics and other pottery items
Thanks to the Natural History Museum of LA County for providing such a beautiful picture.
Fluorescence (floor ESS ence) Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. The most striking example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, while the emitted light is in the visible region, which gives the fluorescent substance a distinct color that can only be seen when exposed to UV light. Rubies, Manganocalcite and Willemite are just a few of the natural minerals that show Fluorescence.
Iridescence (Ear ah des cents) is the play of color often seen when a light source hits a crack, internal fracture or flaws, resulting in the colors of a rainbow. Commercially, in the case of Fire and Ice Quartz, the effect is artificially created producing cracks in rock crystal.
Labradorescence (Labra door essence) is the play of color in metallic hues, especially in labradorite and spectrolite. Blue, gold and green effects are most common but the entire spectrum can be observed. The cause of the schiller is the interference phenomena of twinned layering.
Opaque (oh-pay-k) When you are working with material that is impenetrable to visible light, it is called “Opaque”. The Obsidian to the left is described as Opaque because when it is held up to a light source, no light passes through and it remains unchanged in appearance.
Semi-Transparent (Semi Trans PARENT) Semi Transparent is defined as when a light source is able to move through the piece in some areas which appear more transparent than other areas of denser material in which the light cannot pass though as easily.
Transparent (trans PARENT) The definition of Transparent/Transparency is the property of allowing transmission of light through a material. These pieces are the best to view internal inclusions like mists and veils or phantoms. Often you will hear the term “water clear” used for a high grade transparent crystal.
Translucent (Trans LOO Cent) is defined as allowing light, but not detailed images, to pass through the crystal. Many translucent pieces appear as if a sheer fabric is being held up in front of them. You can somewhat see through the fabric, but the light allowed through is lessened making the piece appear more clouded and muted.