Gold needles — also called karat needles or test needles — are a set of small, bar-shaped reference alloys, each manufactured to a precisely known gold fineness, used in conjunction with a touchstone to assess the karat grade of an unknown gold sample. The method is one of the oldest practical assay techniques still in routine use by jewellers, pawnbrokers, and estate dealers, valued for its speed and its essentially non-destructive character.

Composition and Range

A standard set of gold needles typically spans the commercially significant fineness grades — commonly 10 karat (41.7% gold), 12 karat (50%), 14 karat (58.3%), 18 karat (75%), and 22 karat (91.7%), with some sets including 9 karat for markets where that grade is prevalent. Each needle is an alloy of gold with silver, copper, and occasionally zinc or other base metals in proportions that replicate the colour and acid behaviour of commercial gold alloys at that fineness. Because yellow-gold, white-gold, and rose-gold alloys of the same karat grade contain different proportioning of silver and copper, specialist sets may include separate needles for each colour family, allowing a more faithful colour comparison on the touchstone surface.

The Touchstone Test Procedure

The touchstone itself is a fine-grained, siliceous or lydite stone — traditionally Lydian stone, a black siliceous schist — whose smooth, unreactive surface accepts a thin metallic streak without contamination. The jeweller draws a short streak from the unknown piece and, immediately alongside it, a streak from the needle of suspected matching fineness. A drop of nitric acid (aqua fortis) is applied across both streaks simultaneously. Base metals dissolve rapidly, gold resists; the rate and completeness of dissolution, together with the residual colour of each streak, are compared visually. For higher-karat alloys that resist nitric acid, aqua regia — a mixture of nitric and hydrochloric acids — is substituted. When the two streaks respond identically, the unknown piece is judged to match the needle's fineness.

Accuracy and Limitations

Touchstone testing with gold needles is a comparative, semi-quantitative method. Experienced practitioners can distinguish fineness grades separated by as little as one or two karats, but the result depends heavily on the quality of the needles, the freshness of the acid, the skill of the operator, and the surface condition of the test piece. Plated items, for instance, may yield a misleading streak if the plating is thick enough to dominate the test zone. The method is also sensitive to alloy colour: a yellow-gold needle compared against a white-gold article of nominally equal karat will show an apparent colour mismatch that could be misread as a fineness discrepancy. For these reasons, touchstone testing is best regarded as a rapid screening tool rather than a definitive assay. Where legal or commercial precision is required, X-ray fluorescence (XRF) spectrometry or fire assay remain the authoritative methods.

Sourcing and Maintenance

Gold needles are supplied by assay-equipment manufacturers and jewellery-trade suppliers; reputable sets are accompanied by documentation attesting to their certified fineness. The needles themselves are mounted — individually or in a fan arrangement — in a handle, often of steel or aluminium, for ease of handling. Over time, the tip of each needle accumulates surface oxides and contamination; periodic light filing or burnishing of the test surface is recommended to ensure a clean, representative streak. Acid bottles should be replaced regularly, as diluted or contaminated acid gives sluggish and unreliable reactions.

Historical Context

The use of touchstone and reference alloys to assess gold purity is documented in antiquity; Theophrastus described the touchstone method in the fourth century BCE, and it remained the primary commercial assay tool for precious metals throughout the medieval and early modern periods. The formalisation of karat needle sets as a standardised product is a development of the industrial era, coinciding with the growth of regulated hallmarking systems and the professionalisation of the jewellery trade in the nineteenth century. Despite the advent of electronic and spectroscopic testing instruments, gold needles remain in daily use in small workshops and markets worldwide, particularly where capital equipment costs are prohibitive.