gems, opal, common opal, East Africa, Tanzania

Chrysoprase, the nickel-bearing green variety of chalcedony, has long been valued as a gem material, but its visual similarity to green nickel-colored opal (known as prase opal) has received little attention in the literature. Prase opal from Poland, Tanzania, Australia, and Peru has only been briefly mentioned in past studies. In light of this, the authors undertook a gemological investigation after receiving several samples of nickeliferous opal, particularly from Tanzania. These opals closely resemble chrysoprase in color and display similar diaphaneity, ranging from very translucent to semitranslucent. Both materials are commonly found in association with a brown limonite matrix in the rough, further increasing the potential for confusion. Given this strong likelihood of misidentification, a detailed gemological, chemical, and X-ray analysis of the opal was conducted, with results presented in the study.