Quick answer: Agate, jasper, and carnelian are all chalcedony: quartz made of microscopic crystals. Agate is translucent and banded, jasper is opaque and colored by iron oxides or other minerals, and carnelian is translucent orange-red chalcedony colored by iron. All three are about 6.5 to 7 on the Mohs scale and take a high polish.
Scoop up a handful of tumbled stones and you are likely holding agate, jasper, and carnelian together. They look very different, from clear banded rings to solid brick red to glowing orange, yet they are essentially the same mineral: quartz, grown as crystals too small to see.
Want the full picture? See Quartz: The Complete Guide.
The common base: chalcedony
All three are forms of microcrystalline quartz, silica deposited from water as a dense mass of microscopic fibers and grains. The umbrella term is chalcedony. Our post on agate and chalcedony explains how it forms. What separates the three is how pure the silica is, how light passes through it, and what else got trapped inside.
Agate: banded and translucent
Agate is chalcedony with bands. It usually forms inside cavities, such as the gas bubbles in lava or holes in limestone, where silica is deposited layer by layer from the walls inward. Hold a thin agate to the light and it glows. The bands record pulses of changing chemistry during growth.
Jasper: opaque and full of passengers
Jasper is microcrystalline quartz loaded with other material, especially iron oxides and clay, that can make up a substantial part of the stone. Those impurities make it opaque. They also give it color: red and brown from hematite, yellow and tan from goethite, green from minerals like chlorite. Our post on hematite and goethite covers the iron minerals behind those shades. Jasper often forms in beds or veins rather than in cavities, and it tends to show spots, swirls, and flow patterns rather than neat rings.
A warning worth knowing: the jasper world is full of trade names. Many stones sold as “something jasper” are really other rocks, such as rhyolite, dyed magnesite, or even partly opaque agate, given a catchy name to sell. The name on the label is not always a mineralogical fact.
Carnelian: the orange glow
Carnelian is translucent orange to red chalcedony, colored by fine iron oxide. It has been used for seals, beads, and signet rings for thousands of years because it is hard, carves cleanly, and wax does not stick to it.
Iron-bearing chalcedony can deepen in color when heated, because yellowish goethite converts to reddish hematite. That has been done for centuries, and much of the carnelian on the market today is heated or dyed agate. Natural carnelian tends to show soft, cloudy color variation; heavily dyed stones show color concentrated along bands or cracks. See our guide to dyed and heated stones.
A quick way to sort them
- Light passes through, with bands: agate.
- Light passes through, orange to red, little banding: carnelian.
- No light passes through, earthy color, spots or swirls: jasper.
All three are about 6.5 to 7 on the Mohs scale, which is why they tumble so well and take a bright, lasting polish. Our post on how a tumbled stone is made shows how they get that shine.
