Agates are a type of chalcedony, which is a cryptocrystalline quartz. Agates grow in the voids of rock masses such as lavas, building themselves in layers from the outside-in over millions of years. As the softer, surrounding rock erodes away, the agates remain. Sometimes these agates are themselves eroded and smoothed into pebbles in riverbeds. Other times they can be found as solid nodules that can be sliced and policed, or hollow geodes whose interiors are studded with jagged crystals.

The most common types of nodular agates are marked by distinct, concentric bands. The differences in the colors of the bands in a given agate are due to the impurities in the silica, the basic mineral of the forming rock. Dendritic or landscape agates refer to white gemstones whose inclusions, usually iron-based, resemble a tree or bush growing throughout the stone. Landscape agates are usually cut and shaped so that one of these trees is centered within the finished gem.

The inclusions in moss agates are finer still, caused by manganese (to produce blacks and browns) and chloride (shades of mossy green). And when polished, fire agates reveal clusters of mineral bubbles that appear suspended within the iridescent jewel.

Two other types of agates are created when silica replaces organic materials rather than filling rocky voids. Petrified or agatized wood is formed when wood that has been surrounded by sediments and deprived of oxygen, thus preventing its outright decomposition, is replaced by silica that slowly takes its shape. Agatized coral is formed in much the same way, although these stones are far more rare.

The use of agates in jewelry dates to the end of the stone age; neolithic agate beads believed to be 9,000 years old have been found in Anatolia, present-day Turkey. Thought to provide protection from the evil eye, agates are frequently sliced, cut, and polished into cabochons for rings, bracelets, pins, and pendants. The stone is also carved into cameos.

Art Nouveau and Arts and Crafts jewelers favored agates over traditional gemstones like diamonds, rubies, and sapphires because they appeared more naturalistic. That said, because agates are relatively porous, they are routinely dyed to accentuate their natural colors, thus adding even more differentiation and drama to their naturally occurring bands. In principle, jewelers should disclose whether or not an agate has been dyed, but one way to tell is to put a stone in a sealed plastic bag—when it’s hot out, the agate will sweat out some of its dye.


Best of the Web (“Hall of Fame”)

All About Jewels Dictionary

All About Jewels Dictionary

This incredible reference dictionary on jewelry, from Enchantedlearning.com, is both beautiful and comprehensive. S… [read review or visit site]

Morning Glory Antiques and Jewelry

Morning Glory Antiques and Jewelry

Jewelry collectors, feast your eyes on this internet gem! It's a goldmine of jewelry information featuring all styl… [read review or visit site]

Cathy Gordon's Jewelry Gallery

Cathy Gordon's Jewelry Gallery

With its vast galleries featuring clear images of jewelry and style, this site really covers it all! Divided up by … [read review or visit site]

Jewel History

Jewel History

Since March of 2007, readers of Lori Ettlinger Gross’s Jewel History blog have been treated to her weekly (someti… [read review or visit site]



Clubs & Associations

Other Great Reference Sites

Most watched eBay auctions    

C1800, Antique Georgian Gold Carved Agate Watch Fob Seal Pendant, Semper FidelisBig Antique Victorian C 1890 Sterling Silver Scottish Agate Citrine Brooch PinSuperb Antique Victorian C1890 9 Ct Gold Scottish Agate Citrine Brooch PinVictorian Scottish Banded Agate And Cornelian BroochAn Antique Scottish Silver And Agate Anchor Brooch, Hallmarked C.1900Vintage Art Deco C 1930 Sterling Silver Green Agate Chrysoprase BraceletSuperb Rare Antique Victorian 1890 9 Ct Gold Scottish Eye Agate Brooch PinAntique Georgian Gold Cased Carved Crested Agate Fob Seal Pendant, Virtute QuiesFine Large Silver Gilt Banded Agate Victorian Style Pendant Necklace, 925C1898, Antique Victorian Solid 9ct Rose Gold Agate Spinning Watch Fob PendantStunning Antique Victorian Scottish Agate Bow Design Pebble Bracelet 1880-broochAntique Art Deco Solid 10k Gold Filigree Fire Agate Stone Ring GorgoeusC1908, Antique Edwardian Solid 9ct Rose Gold Agate Spinning Watch Fob PendantVictorian Scottish Carved Natural Banded Agate Dropper Earrings-14k Solid GoldSuperb Antique Victorian C1890 9 Ct Gold Scottish Scenic Moss Agate Brooch PinVictorian Scottish Agate, Citrine And Silver Penannular BroochAntique Solid Silver Scottish Agate Pebble Brooch Heraldic Sheild Coat Of ArmsVintage Art Deco C1930's Silver Scottish Agate Champagne Citrine NecklaceSuperb Mint Condition Antique Victorian Scottish Agate Fossil Silver Brooch PinFine 18th Century Georgian Pinch Beck Agate Cross PendantBig Old Handmade 24k Gold Fire Agate Gemstone Pendant 18" Gold Chain NecklaceSuperb Large Antique Hm 1916 Sterling Silver Scottish Agate Citrine Brooch PinLarge Antique Vintage Sterling Silver Mounted Banded Agate Brooch/pinVintage Lea Stein French Pin Swallow Swift Bird Fabby Ivory/torty/banded AgateVictorian Scottish Agate And Silver BroochFine Victorian 14ct Gold 3 Tier Agate Large Drop Dangly Heavy Earrings 14k 585Superb Big Antique Victorian C 1890 Silver Scottish Agate Dirk Kilt Pin BroochAntique Scottish Agate Bracelet Silver Mounts & Silver Padlock Antique Early C1900s - Art Deco Chinese Carved Hardstone Cameo Ring. Jade? Agate Antique Victorian C 1890 Sterling Silver Scottish Agate Pebble Brooch PinAntique Solid Gold Scottish Dirk Pin Brooch Banded Agate & Amethyst 1900sAntique Victorian Scottish Silver Brooch With Agate, Citrine & Amethyst.Antique Victorian14k Gold Agate Cameo Brooch Pendant 50mm X 38mm #jc53Vintage Hand Knotted Scottish Agate Cornelian NecklaceBig Vintage Art Deco C1930's Silver Scottish Agate Champagne Citrine Brooch PinVintage Signed Banded Agate Sterling Silver CuffllinksKendra Scott Pretty Kathleen Antique Silver Platinum Druzy Agate Chain BraceletAntique Victorian 10k Gold & Scottish Banded Agate Brooch Signed Wjco C1850Antique Victorian C 1890 Sterling Silver Scottish Agate Pebble Brooch PinA Victorian 9ct Gold & Bullseye Agate & Enamel Mourning Brooch 11.8 GramsSuper Lot Of 15 Antique & Vintage Jewellery Items Agate Silver Brooches Locket +Rare Antique Victorian C 1886 Silver Scottish Agate Liverpool Clipper Brooch PinAntique Sterling Silver Mounted Banded Agate Heart BroochLarge Vintage Circa 1930's Scottish Agate Paste Citrine Plaid Celtic Brooch PinBig Antique Victorian Scottish Malachite Agate Lovers Knot Silver Pebble BroochVintage Jewellery Sterling Silver Blue Agate Art Nouveau Design BroochVintage Jewellery Blue Banded Agate Glass Bracelet (spinx)Vintage Jewellery 925 Sterling Silver Moonstone & Red Carnelian Agate PendantVintage Jewellery Very Attractive Sterling Silver And Moss Agate Cabochon RingSuperb Art Deco Hm 1924 Silver Scottish Agate Pebble Bangle Bracelet Brooch PinExceptional Victorian Antique 14k Gold Moss Agate Handmade Pendant! Nr!Victorian Scottish Banded Agate BroochAntique Victorian 1890 Sterling Silver Scottish Agate Citrine Thistle Brooch PinVintage Scottish Silver, Agate And Citrine? Brooch.Vintage/antique Jewellery Silver Scottish Agate Clip On EarringsKendra Scott Kimmy Antique Plated Brass Faceted Band Agate Pendant NecklaceAntique Victorian C1890 9 Ct Gold Scottish Carnelain Agate Brooch PinLarge Antique Victorian C1890 Gilt Metal Scottish Carnelian Agate Brooch PinVtg Sterling Silver - Navajo Zuni Native American Agate Ring Size 7.5 - 10gVintage Jewellery Pretty Double 2 Row Connemara Marble Agate Glass Bead Necklace