Chrysocolla is a hydrous copper silicate mineral that forms from the weathering of copper minerals. In chalcedony, chrysocolla usually occurs as cryptocrystalline botryoidal aggregates with other copper minerals. In some samples, these chrysocolla/copper mineral aggregates form very interesting patterns; in others, the aggregates are very fine grained and well integrated with the chalcedony.

The latter samples appear homogeneous with no visible inclusions. In fact, inclusions of chrysocolla together with other copper minerals (such as malachite) within the chalcedony provide the best evidence of natural origin.   In addition, chrysocolla with visible inclusions typically is not dyed, because of the premium placed on stones with attractive internal assemblages. If a piece of “clean” chalcedony is dyed with cobalt, characteristic absorption lines (triplet at 620, 657, and 690 nm) may be seen with a handheld spectroscope. However, if the stone is dyed with a copper solution, UV-Vis-NIR spectroscopy is necessary to identify the presence of treatment.

UV-Vis-NIR spectra were recorded on 29 untreated and 7 copper-treated samples. A typical UV-Vis-NIR absorption spectrum (250–2500 nm) of natural chrysocolla chalcedony shows four distinct broad bands. A broad band covering 527–1176 nm (centered at ~721 nm) is due to the crystal-field effect of the Cu²⁺ ion in the chrysocolla lattice. A band around 1300–1700 nm correlates to total OH content. The molecular water content is represented by a band at 1800–2100 nm. A band at 2128–2355 nm represents structurally bonded OH. The same four broad bands were observed in the Cu-treated samples. Although the concentrations associated with the four bands can be calculated if the path length and the absorption coefficient are known, separation of the two materials was accomplished by a simpler approach: comparing the ratios of the areas under the Cu²⁺ (527–1176 nm) band and the structurally bonded OH (2128–2355 nm) band. All of the chalcedony colored by chrysocolla had a ratio between 7 and 44, whereas all the samples dyed with copper solutions had a ratio from 0.5 to 3.

This technique is very sensitive to the level of hydration in minerals and glasses. Opal can be readily distinguished from chalcedony using this method, which may be helpful for the separation of other hydroxyl-containing gem materials as well.