Chalcedony is one of the oldest, most abundant, and most popular gem materials. It was named for the ancient Turkish seaport of Chalcedon (now Kadikoy). It is relatively affordable and comes in a very wide range of colors and patterns – two factors that make it popular with gem and mineral collectors. In jewelry, it is most often seen in beads, cabochons, tablets, and carvings.

Chalcedony is cryptocrystalline quartz that can range from semitransparent to opaque. Its many varieties are based on differences in color, color distribution, transparency, and phenomena.

Jasper is a general variety name for semitranslucent to opaque chalcedony of any color or combination of colors other than solid black chalcedony or specially named varieties like bloodstone. Also called heliotrope, bloodstone is semitranslucent to opaque dark green with red to brownish red spots.

Chalcedony is an aggregate with a spot refractive index (RI) of 1.53 or 1.54. Its birefringence, which is usually not detectable, is 0.004. Chalcedony is generally inert to UV radiation, but might fluoresce a weak to strong yellowish green. Its specific gravity (SG) is 2.60, and its fracture is normally conchoidal but sometimes granular.

Once a material has been identified as chalcedony, its variety must also be determined. Chrome chalcedony, dyed green chalcedony, chrysocolla-in-chalcedony, chrysoprase, and prase are all chalcedony varieties that come in green hues.

Chrome chalcedony is rare, intensely green, and, as its name implies, colored by chromium. It sometimes contains tiny black chromite crystals, which help distinguish it from dyed green chalcedony. Its slightly higher SG (usually around 2.60, but sometimes above 2.62) might also help separate it from dyed green chalcedony.

Chrome chalcedony’s spectrum has a vague band around 645 nanometers (nm) and a distinct line at 684 nm. The spectrum for dyed green chalcedony has a similar appearance. Also, both materials can look pinkish or reddish through a color filter.

Chrome chalcedony resembles green jadeite, but jadeite’s RI and SG are both much higher.

Chrysocolla-in-chalcedony is translucent to semitranslucent and a light blue or green-blue color. The gem is colored by inclusions of the copper mineral chrysocolla. With enough chrysocolla inclusions, it can resemble fine turquoise, though chrysocolla-in-chalcedony is usually more transparent.

Distinct differences in RI and SG will separate chalcedony from turquoise. Chalcedony’s RI is 1.54, and its SG is 2.60. Turquoise’s RI is 1.61 to 1.65, and its SG is 2.76. If there is a high chrysocolla content, chalcedony takes on the characteristics of pure chrysocolla, with a Mohs hardness of 2–4 and an SG near 2.20.

Chrysoprase chalcedony’s natural color is light to medium yellowish green. It is colored by nickel-rich clay inclusions. It is possible to confuse chrysoprase with jadeite, jade substitutes, prase, opal, translucent emerald, bowenite serpentine, dyed green chalcedony, and glass. Chrysoprase’s 1.53 or 1.54 RI and its SG (usually greater than 2.62) separate it from most of these look-alikes. Prase has a grayish green hue that sets it apart from chrysoprase.

Chrysoprase shows no reaction to a color filter, while dyed green chalcedony usually appears pinkish or reddish. In addition, dyed green chalcedony shows vague lines at 645 and 670 nm, while chrysoprase might show only a cutoff between 660 and 700 nm. Also, the color of dyed green chalcedony often looks unnatural. It is generally strongly bluish green, while natural chrysoprase’s green is much more yellowish.