Prase Opal — The Nickel-Green Common Opal
Prase Opal — The Nickel-Green Common Opal
An apple-green to deep-green common opal coloured by trace nickel, found alongside chrysoprase in Tanzania, Madagascar, and Australia
Prase opal is a green variety of common opal coloured by trace nickel, ranging from pale apple-green through saturated grass-green to deep moss-green. It lacks play-of-colour, placing it firmly in the common opal category alongside potch, hyalite, and milky white opal, and is distinguished from precious opal by the absence of the diffraction phenomenon that defines black, white, and crystal opal. Prase opal sits next to chrysoprase in the green-silica family — both are nickel-coloured cryptocrystalline silicas — and the two materials are often found in the same geological setting.
Composition
Prase opal is hydrated silica, SiO2·nH2O, with the green colour produced by trace nickel substituting in microscopic inclusions of nickel-bearing phyllosilicates such as pimelite or willemseite, or by nickel adsorbed onto the silica framework. Water content runs in the typical opal range of 6 to 10 percent. Hardness is 5.5 to 6.5 on the Mohs scale, lower than chrysoprase, with specific gravity 1.98 to 2.20 and refractive index near 1.45. The material is generally translucent to opaque, with the better specimens approaching the translucency of low-grade chrysoprase.
Sources
Tanzania has been the principal source of fine prase opal since the early 2000s, with material from the Haneti and Itiso districts of central Tanzania reaching the international market in significant quantity. Tanzanian prase opal is typically vivid apple-green to deep grass-green and translucent enough for cabochon work that shows light through the thinner sections. Other reported sources include Indonesia, Madagascar, and parts of Western Australia, although Tanzanian production dominates.
The material occurs in weathered nickel-laterite deposits where groundwater leaches nickel from underlying ultramafic rocks and deposits silica gel in fractures and voids, in much the same geological environment that produces chrysoprase at Marlborough in Queensland and at Szklary in Poland.
Identification
Prase opal differs from chrysoprase in refractive index (lower, near 1.45 versus 1.53 to 1.54 for chrysoprase), specific gravity (lower), hardness (softer), and infrared spectrum (water-bearing absorption distinct from the chalcedony fingerprint). Microscopic examination shows the granular silica gel structure characteristic of opal rather than the cryptocrystalline quartz fabric of chalcedony. Some prase opal has been confused commercially with chrysoprase; laboratory testing resolves the species in seconds.
Treatments are uncommon. Most prase opal reaches the market in untreated form, although some material is dyed to deepen colour and should be disclosed under FTC and CIBJO guidelines.
In the trade
Prase opal is fashioned as cabochons, beads, and tumbled forms, with the better Tanzanian material commanding prices broadly comparable to mid-grade chrysoprase. The colour is the principal value driver — saturated apple-green stones with even distribution and good translucency are the desirable end of the market — and clean material without visible matrix or fractures is preferred. Designers favour the colour for accents in silver and yellow gold, where the cool green sits well against warm metal.
Care
Prase opal is more durable than precious opal but still benefits from opal-protocol care: avoid ultrasonic and steam cleaning, prolonged solvent exposure, and dehydrating storage. Mild soap, warm water, and soft cloth are the standard cleaning regime. The hardness suits the material to pendants, earrings, and protected ring settings rather than exposed daily-wear designs.