Agate Terms and Definitions

Agate hunters have their own unique lingo. The definitions below are from "The Lake Superior Agate" by Scott F. Wolter.


Term Definition
abrasion Scraping action induced by glacial processes; also the tumbling action of streams and beaches.
accessory mineral Those mineral constituents of a rock that occur in such small amounts that they are disregarded inits classification and definition.
agate A waxy variety of crytocrystalline quartz in which the colors occur in bands,clouds,or distinct groups.
aggie ball A relatively round agate, usually small, with no banding or internal features visible.
all-timer An agate of exceptional quality, very rare and very valuable.
amygdule A gas cavity or vesicle in volcanic rocks,filled with secondary products suc as zeolites, calcite,and silica minerals.
amethyst A purple or bluish-violet variety of quartz used as a gem.
basalt Dark, volcanic rock composed of fine grained minerals.
bleached agate Whitening or loss of color due to prolonged exposure to sunlight.
botryoidal texture Natural habit of crystal growth resembling the form of a bunch of grapes.
calcite One of the most common minerals;the principle constituent of limestone.
carbon dioxide A colorless,oderless,nonpoisonous gas that is a minor but normal part of ambient air.
Cenzoic The latest of the four eras of geologic time,extending from the close of the Mesozoic (65 million years ago) to the present.
chalcedony Cryptocrystalline quartz, the material of agate and chert.
chatoyance Having a luster resembling the changing luster of the eye of a cat, generally a property of translucent materials containing parallel fibrous structures capable of scattering light.
colloid A state which exists between a true solution and a suspension, arises because the particles are so small that electrical and other forces which tend to support them in the dispersion medium are greater than the gravitational forces tending to cause them to settle.
composition An aggregate, mixture, mass, or body formed by combining two or more substances;the chemical constituents of a rock or mineral; the mineralogical constitution of rock.
conchoidal fracture A type of rock or mineral fracture giving smoothly curved surfaces, characteristics of quartz.
cross-section A profile portraying an interpretation of a vertical section of the earth explored by geophysical or geological methods.
crystal impression A mold or cast left protruding into an agate, Usually made by calcite before agate formation.
crystallization The process through which crystalline phases separate from gas,liquid solution, or rigid solution.
decorative stone Rock used in landscaping adorning homes,businesses, ect.
deposition The precipitation of mineral matter from solution as the deposition of agate or vein quartz.
drift Any rock material, such as boulders, till, gravel, sand or clay, transported by a glacier and deposited by or from the ice, or in water derived from the melting of the ice.
drop Accumalation of rock at the base of a gravel pit wall or bank.
erosion The group of processes by which earthy or rock material is loosened or dissolved and removed from any part of the earth's surface.
ettringite A common hydration mineral product of portland cement that typically forms within the cement paste.
eye The perfectly round, circular banded pattern found on some agates; rare.
face A term applied to a well exposed banded area on an agate.
field stone Glacially deposited gravel and rock exposed at the surface.
fill hole That area or areas where solutions entered and exited the vesicle.
floating bands Bands of chalcedony bound on both sides by clear crystal quartz.
fortification A term applied to a a pattern of agate banding often resembling the enclosing nature of a fort.
fracture The manner of breaking and appearance of a mineral when broken, which is distinctive for certain minerals.
frost action The weathering process caused by repeated cycles of freezing and thawing.
gelatin Jellylike material formed by coagulation of a colloidal dispersion.
gemstone A general term for any precious or semiprecious stone.
geode A hollow, globular body, with an interior lining of inward-projecting crystals.
glacier A mass of ice with definite lateral limits, with motion in a definite direction and originating from the compacting of snow by pressure.
holocene Recent, that period of time since the last ice age.
hunt To spend untold hours searching for that elusive Lake Superior agate is afoot.
husk A general term applied to the unabraded exterior of an agate.
hydrostatic pressure The pressure exerted by water at any given point in a body of water at rest.
interbedded Occurring between beds, or lying in a bed parallel to other beds of a different material.
iris The rainbow effect sometimes produced when sunlight is refracted through a thinly cut slab of agate.
iron-oxide Any mineral containing iron and oxygen, including hematite, magnetite, goethite, and limonite.
juvenile waters Water that is derived from the interior of the earth and has not previously existed at atmospheric or surface water.
lacy banding The undulating pattern of chalcedony bands deposited over the tops of quartz crystals within an agate.
lapidary A skilled work of cutting and polishing gems or other stones.
laurentide ice sheet A mass of continental ice centered, during the Pleistocene Epoch, over what is now Hudson Bay.
limonite A mineral; field term for a group of brown, amorphous, naturally occurring, hydrous ferric oxides.
magma A molten rock, formed within the crust or the mantle of the earth, which may consolidate to form an igneous rock.
manganese oxide Any mineral containing manganese and oxygen.
meltwaters Rivers and streams of melted glacial ice.
mesozoic An era ranging in time from 230 to 65 million years ago, a duration of 165 million years.
microstratigraphy microscopic features within agates.
mineral A structurally homogenous solid of definite chemical composition, formed by the inorganic processes of nature.
moss agate A variety of agate containing solid, moss-like masses of manganese-oxides entombed by massive or banded chalcedony.
original banding Exposed banding unaltered by weathering or oxidation.
outwash Drift deposited by meltwater streams beyond active glacier ice.
overburden A general term for all glacial deposits, including till, moraine, outwash, ect.;also regolith, alluvium, ect. in unglaciated areas.
oxidation A process of combinng with oxygen;removal of one or more electrons from an ion or an atom.
painted agate A heavily oxidized and stained agate that appears painted.
paleozoic An era ranging in time from 600 to 225 million years ago, a duration of 375 million years.
paradise beach agate A type of agate originating from Paradise Beach located on the North Shore of Lake Superior in Minnesota;many contain small stringers of copper.
peeled agate An agate with fracturing between bands along the banding plane.
percussion cracks Crescent-shaped cracks caused by violent contact with other rock, on rocks with conchoidal fracture.
permeable Having a texture that permits water to move through it perceptivly under the head differences ordinarily found in sub-surface water.
pick To hunt for agates.
pleistocene The earlier of the two epochs comprising the Quaternary Period.
pit Any depression on husk of agate; also used to mean gravel pit.
precambrian All rocks formed more than 600 million years ago.
quartz A mineral;one of the most common minerals on earth.
quartz ball An agate composed primarily of clear crystal quartz.
Quaternary Period The most recent time period, beginning 2 million years ago and continuing into the present.
rifting event The formation of a deep fracture or break in the earth where magma upwellings occur together with spreading along the rift and the creation of new volcanic rock.
rough stone An untreated, unpolished agate.
saginite An acicular variety of rutile occurring in groups of crystals crossing at 60 degrees and often enclosed in quartz or other minerals.
similar banding A group of two or more bands of the same thickness and composition.
shadow agate A type of agate that exhibits the optical effect created by the perception of depth between parallel bands of chalcedony.
skin Another name for the intact surface of the agate that was in direct contact with the host lava; the husk.
smokey quartz A smoky, black- to brown-colored crystalline varietyb of quartz caused by exposure to natural radiation.
solid agate An agate banded completely with no clear quartz present.
spherical Shape of a fragment approaching the form of a sphere.
stain Refers to color due to oxidation of iron-minerals.
stratified Formed or lying in beds, layers, or strata.
stringer A narrow vein or irregular filament of mineral traversing a rock mass of different material.
sugar quartz A term for clear macrocrystalline quartz named for it's sugary appearance.
Superior lobe The lobe of ice that followed the depression or trough of Lake Superior into Minnesota, carrying Lake Superior agates.
till Unsorted debris deposited directly from glacial ice.
topography The physical features of a district or region;the relief and contour of the land.
translucence Admitting the passage of light, but not transparent.
treated An agate that has been altered in appearance without physically defacing or removing any material.
truncating bands Banding that appears cut off but actually is pinched so fine as to appear cut off.
tube agate A variety of agate with usually parallel, linear projections of mineral matter into the vesicle.
tumble To polish agates inside rotating drum with polishing grit.
vesicle A small cavity in an igneous rock, formed by the expansion of a bubble of gas or steam during the solidification of the rock.
volcanic Produced, influenced, or changed by a volcano or by volcanic processes.
washout Small-scale exposure of gravel and rock washed out by rain or meltwater.
waterlevel agate A variety with flat parallel banding topped with fortification pattern or clear quartz.
weathering The group of processes, such as the chemical action of air, rainwater, plants, and bacteria in addition to the mechanical action of changes of temperature, whereby rocks on exposure to the weather change in character, decay, and finally crumble into soil.
Wisconsin Glaciation The last of the four classical stages in the Pleistocene in North America.