IDENTIFYING AGATE AND JASPER
Historically by definition true agate is a transparent form of chalcedony with only concentric banding but today the term agate is commonly used to include chalcedonies with many different kinds of mineral inclusions, patterns and colours. Agates of many different types may be found throughout the world.
Agate is present primarily in volcanic flows and forms veins, masses nodules.
Iris agate isn't a particular type of agate, but rather a visual phenomenon that occurs rarely in certain agates. The iris effect is usually found in colourless banded nodular agates, often with a quartz filled centre. The iris effect is caused by extremely fine banding that diffracts light as it passes through the stone and produces a spectrum of rainbow colours. The iris effect is not visible to the naked eye under ordinary conditions, and the agate must be cut into very thin slices and viewed in front of a light source at the correct angle to see the fire within
Iris agate is a specimen only
Jasper is also a form of Chalcedony that is opaque, tough, fibrous and massive with a greater density of mineral inclusions such as iron oxides and earthy clays than agate. Jasper can occur as nodules or veins in bedded sedimentary or metamorphic deposits, and altered igneous rock deposits. Jasper is often found in conjunction with agate and the two are often intermingled (called jasp-agate), creating colourful and striking specimens.
Jasper has many of the same type of patterns as agate although they are usually not quite as fine as those seen in agate due to the more granular and less transparent nature of jasper.
As with agates there is an almost infinite variety of jaspers that are found throughout the world.
Jasp-agate is a jasper that falls somewhere between agate and jasper due to the high amount of chalcedony present in the stone.
Jasp-agates tend to be very colourful, hard and well patterned and are a highly desirable lapidary stone
Aggregate Stalks Finger like structures in a row emanating from one direction one plane
Banded agates and jaspers are recognised by their parallel concentric banding and nodular shape. Most banded agates and jaspers are not overly colourful, usually occurring in subtle shades or white, gray or tan. There are exceptions though there are certain locations around the world where vividly coloured stones can be found with hues of vibrant reds, pinks, purples, yellow etc
Brecciated Agate and Jaspers are stones in which previously broken angular pieces of agate or jasper are re-cemented with chalcedony to form a new type of stone. The re-cemented pieces are randomly arranged and the internal pattern often has a jagged mosaic look with clear lines of demarcation between the more solid fragments.
Clear transparent or translucent The clear stones only apply to Agate. Jasper is opaque
Cloud stones have a cloud like wispy patterns through the stone.
Dendritic is a type of stone that displays sharply defined, two dimensional tree like branching patterns within the stone.
The very best dendritic agate has the dendrites floating in the clear chalcedony.
Dendritic jaspers tend to be earthy colours such as tans. browns and creams with black to brown dendrites. Some dendritic jaspers tend towards a form of opalite
Dot consist of random floating solid round dots of varying size and colour on a plain background.
Eye or Orbicular
Eye agates contain spherulites or orbs of one or more concentric round rings that resemble eyeballs. An eye agate with one large eye is often called a bullseye agate.
Orbs can range from small dots or spherules to larger undulating patterns of concentric, overlapping rings.
Fossiliferous contains fossil remnants of ancient plants, marine life, shells or skeletons. Colours may include gray, white, black, brown, green, and red.
spherulites are small, rounded bodies that commonly occur in vitreous igneous rocks. They are often visible in stones as globules about the size of millet seed or rice grain, with a duller lustre than the surrounding glassy base of the rock, and when they are examined with a lens they prove to have a radiate fibrous structure.
Flame is generally a vein stone with patterns shaped like flames of fire. The flame patterns generally run along the outside edges of the stone with the flames pointing inward, with the centre of the stone usually fairly clear and transparent.
Floater small floating banded section in the centre of a nodule surrounded by chalcedony
Flower or Bouquet
Flower is a nodular or veined stone with inclusions that look like flowers, bouquets, or round puffy plumes on a clear chalcedony background.
Fortification differs from banded in that the bands are much more complex and irregular with scallops, angles, and sharp curving patterns and often containing other inclusions. Fortification is generally gray or white with other mineral inclusions making up the distinctive colour patterns.
Lace is a stone that has an overall lacy pattern that may include eyes swirls banding scallops and zigzags. When the lace pattern has over 90% off zig zags it is then called lattice
Lattice is angular bladed sections in a zigzag or lattice like pattern within the stone.
Moss is a vein or nodular stone that contains colourful and delicate inclusions, that resemble sea weed, moss, strings, or filaments on a clear chalcedony background. Moss agate can occur in a variety of colours including green, red, yellow, orange, black and white, The moss inclusions are created by different mineral impurities such as manganese, chlorite, or iron oxides,
Mottled stones have an irregularly marked or blotchy appearance. It can come in a wide array of colours including red, green, brown, yellow, orange and gray.
Plume is a vein or nodular stone that displays graceful, three dimensional , feather or plume like inclusions on a clear chalcedony background. The plume patterns may consist of only one single plume or a small grouping of plumes. The plumes must be distinctive, individual plumes similar to and ostrich feather. Plumes can appear in a variety of colours and may have sub-metallic lustre due to the minerals such as pyrolusite, goethite, or manganese.
Sagenite is a vein or nodular crystal with needle like inclusions occurring loosely dispersed or collectively in sprays or fan shapes.
Sagenite has a clear chalcedony background where the needles are uninterrupted and appear to be floating freely. Sagenite needles are composed of a variety of minerals, including rutile, manganese, chlorite, actinolite or goethite, and are often pseudomorphic replacements of other minerals such as barite or aragonite
Scenic or Picture
Picture or scenic crystals are some of the most popular gem materials. Picture crystals occur in a large array of colours and patterns that resemble skies, mountain vistas, desert landscapes, hills and valleys and forest horizons. Colours include warm tones of tan, gold, yellow, blue, green, and browns all swirled together in a strikingly outlined picturesque scenes.
Snowflake is a vein or nodular crystal that has little floating white flecks or flakes in clear chalcedony that looks like snowflakes. Snowflakes tend to occur in banded agates and jaspers, but can appear in almost any of the other types. In some dendritic agates and jaspers, the snowflakes are coloured black and brown
Spider-web is a criss-cross or woven type of pattern overlaying a solid colour background. spiderweb jaspers can occur in colours of green, red black cream and brown. Some spider web patterns are small and delicate while others are large and flashy these ones are also called picasso stones
Tube agate is a vein or nodular agate with internal pseudomorphic replacement of crystals or needles that create a stalactite or tube like formations within the stone. It looks like a worm has crawled around inside the rock. When the agate is cut parallel to the pattern it shows long finger like tubes, and when cut across the tube an eye pattern is shown.
Waterline Agate is a nodular or vein agate that exhibits straight rows or coloured lines, usually white on a clear chalcedony background/ Waterline agate can occur with other agate patterns, such as banded, dendritic, or moss types. Waterline agate is also known as Onyx Agate or Uruguay banding
A botryoidal texture or mineral habit is one in which the mineral has a globular external form resembling a bunch of grapes as derived from the Greek. This is a common form for many minerals particularly hematite where it is the classically recognised shape. It is also a common form of goethite, smithsonite, fluorite and malachite. This includes chrysocolla.
chatoyancy or chatoyance or cat's eye effect, is an optical reflectance effect seen in certain gemstones.
Coined from the French "œil de chat," meaning "cat's eye," chatoyancy arises either from the fibrous structure of a material, as in Jaspers, or from fibrous inclusions or cavities within the stone, as in cat's eye chrysoberyl.
The effect can be likened to the sheen off a spool of silk: The luminous streak of reflected light is always perpendicular to the direction of the fibres. For a gemstone to show this effect best it must be cut en cabochon, with the fibres or fibrous structures parallel to the base of the finished stone. Faceted stones are less likely to show the effect well.
Druse or druzy is a coating of fine crystals on a rock fracture surface, vein or within a vug or geode.
Fire Displays shimming iridescence within surface layers
Fluorescent exhibits bright colours under ultraviolet light in darkness
Iris rock when cut thin and backlit displays iridescence within its bands
Opalescence displays fiery flashes of colour within surface layers
When banded and fortification agates have very tight banding a phenomena occurs called parallax, this is a visual effect that resembles a shadowy movement within the stone's bands when is is slowly rotated in the light.
Schiller, from German for "twinkle", is the metallic iridescence originating from below the surface of a stone, that occurs when light is reflected between layers of minerals.
Geodes are formed by Chalcedony that is deposited in Rhyolite cavities leaving hollow centre. Geodes are frequently lined with beautiful crystal formations, including amethyst, and are primarily cut in half and polished as display specimens. some small geode slices are used in jewellery.
Milky Quartz vein in Massive Quartz Mineral
massive An amorphous structure thick and dense often in large solid chunks or seams
Nodule rock with and agatised centre almond, oval or football shaped
Polyhedral is a sharply geometric or angular shaped nodule
Pseudomorphs are formed when minerals, usually in the form of chalcedony replace another mineral or organic substance and the shape is retained of the original object, but its chemical composition has been permanently altered. The item replaced maybe animal such as dinosaur bones or marine coral; plant such as fossilised wood or fossilised ferns; or other minerals, such as barite or aragonite crystals that are replaced and filled by chalcedony. Other crystals such as calcite, pyrite, azurite, malachite, and geothite may also form as replacement materials.
Thunder egg rock with a solid agatised internal centre cavity surrounded by a matrix, it is usually round
Milky Quartz Vein
vein rock formed within a crack or fissure flat or rectangular shaped