Coated Quartz

In the Aqua Aura-coated quartz G&G examined in 1990, refractive index, birefringence, optic character and specific gravity all matched published values for quartz, so routine readings only confirm the substrate; magnification was the key to spotting the coating (dark outlines on facet junctions, whitish junctions where the film had worn away, uneven facet colour, strong iridescence in reflected light). Blue coated quartz resembles cobalt-doped blue synthetic quartz and can be separated by magnification, the Chelsea filter reaction and the absorption spectrum. GB/T 16553—2017 lists, for coated rock crystal, abnormal surface lustre and local peeling of the film under magnification, possibly abnormal refractive-index readings, and characteristic film peaks in infrared and Raman spectra. Whether a stone is coated must be decided by an accredited laboratory.

Scope: Identification points; testing must be done by an accredited laboratory, and no self-test thresholds are given.

GB/T 16552—2017 defines coating as laying a thin film on a gem's surface by painting, plating or similar means, to change its lustre or colour, create a special effect or protect it, and its Annex B Table B.1 classes coating of rock crystal as a treatment. Coated quartz is rock crystal treated in this way.

Scope: Applies to the standard's definition of coating and its treatment class for rock crystal; it does not cover specific film materials or process parameters.

The Fall 1990 Gem News column of Gems & Gemology recalled that 'Aqua Aura' quartz consisted of colourless quartz crystals and clusters given a thin gold film on their outer surfaces, showing the blue to greenish blue transmitted colour of the gold along with a superficial thin-film iridescence; by then the treatment was being applied commercially to faceted gems, topaz as well as quartz.

Scope: Applies to the gold-film Aqua Aura treatment described by G&G in 1990; the sources do not say which film materials other aura names use.

GIA's introduction to gem treatments notes that quartz is occasionally given a metal-oxide coating for colours natural quartz seldom shows; vapour deposition can lay such a film on quartz crystals or on stones that are already faceted, and the thin layer changes their colour.

Scope: An overview of coated quartz from one educational source; it gives no process parameters.

G&G's 1990 tests on Aqua Aura specimens found that a heated thermal-reaction tester and dilute hydrochloric acid had no noticeable effect on the film and a Mohs 6 hardness point could not scratch the coated quartz, so the authors judged the treatment fairly durable; but because the film is only on the surface, recutting or repolishing would take off part or all of it.

Scope: Results from tests on a few Aqua Aura specimens in 1990; not a statement about the durability of every coated quartz.

GIA's introduction to gem treatments points out that surface films of every kind are often softer than the stone beneath or poorly bonded to it and so scratch easily, especially at facet edges and where facets meet; hard or abrasive objects should be kept away from coated stones, which are best wrapped in something soft and stored dry when not worn.

Scope: The source writes about coated gems in general, not about coated quartz specifically.

The care clauses of the CIBJO Gemstone Book state that coatings on stones are often easily taken off by solvents, heat or abrasives, so coated stones should be kept away from all three, and that they are not suitable for recutting or repolishing.

Scope: A general CIBJO care rule for coated stones, not one written for coated quartz specifically.

(Reference) iridescence in natural quartz comes from thin-film interference or from repetitive substructures acting as diffraction gratings, and many fractures in minerals show iridescence through thin-film effects; in coated quartz the iridescence comes from a film applied to the outer surface (G&G in 1990 called it a superficial thin-film iridescence). The two can look alike, and neither the name nor the look can replace testing.

Scope: Explains how the two kinds of iridescence arise; it is not used to judge any specimen.

Gems & Gemology notes that, as demand for naturally iridescent quartz grew, iridescent quartz has also been made artificially by depositing thin films of various metals (as with "aurora quartz"). GIA's treatment guide likewise says quartz is occasionally coated with metal oxides, applied by vapour deposition, to give colours rarely seen in natural quartz.

Scope: Neither source names titanium or the trade name "titanium aura"; they establish only that metal or metal-oxide films are deposited on quartz.

GIA's laboratory has documented titanium used as a colour coating on a gem: in 2014 it reported tanzanites whose pavilions carried a titanium-bearing coating, its first such case for tanzanite. On those stones the coating had worn away along facet junctions and in small chipped areas.

Scope: The material is tanzanite, not quartz. This shows only that titanium coatings on gems have been recorded by GIA; it is not a record of "titanium aura" quartz.