A stone or bead is rarely worn bare. It is set in a ring, a pendant or a cap, and the metal of the setting is chosen for tradition, cost and durability. The laboratory tests the stone or bead, not the metal — but the setting affects what can be tested, and a buyer should understand both what the metal is and what it can conceal.
The metals
Gold. Yellow gold is alloyed with copper and silver; the purity is stated in karats (22 kt is 91.7% gold, 18 kt is 75%) or fineness (916, 750). Hallmarking under the Bureau of Indian Standards scheme states the purity on the piece. Soft in high purity, durable in 18 kt.
Silver. Sterling is 92.5% silver (925). Tarnishes; soft; inexpensive; the traditional metal for Rudraksha caps and many astrological settings.
Panchdhatu. "Five metals" — traditionally gold, silver, copper, iron and zinc (or lead, or tin, depending on the source), in proportions that vary between makers. In practice the alloy is mostly copper and zinc (brass-like) with small or nominal amounts of gold and silver. Prescribed in some astrological traditions for certain stones; its composition is not standardised or hallmarked, and "panchdhatu" on a listing tells the buyer little about what the ring is actually made of.
Ashtadhatu. "Eight metals" — the five above plus, variously, tin, lead and mercury or others. Used in some traditions for idols and rings. As with panchdhatu, composition is unregulated.
Copper. Prescribed in some traditions for particular stones; corrodes and turns skin green.
Base-metal alloys, plating, and "German silver". Inexpensive settings that may be plated with gold or silver over brass or nickel. Plating wears; nickel can cause skin reactions.
What tradition prescribes
Astrological practice often specifies a metal for each stone — gold for yellow sapphire and ruby, silver for pearl and moonstone, panchdhatu or gold for blue sapphire and emerald, and so on — with variations between practitioners and texts. The laboratory takes no position on these prescriptions. It notes that a setting's metal does not change what the stone is, and that testing should precede setting.
What a setting can hide
The back of the stone. A closed-back setting hides the pavilion, where the refractometer needs a facet, where filling and dye show, and where a doublet's join sits. Open-back settings allow more examination.
The girdle. Where diamonds carry inscriptions and where composite stones show their layers.
The poles of a Rudraksha. Caps cover the crown and base, where carved grooves fail to meet. Capped Rudraksha: what caps can hide.
Weight. A set stone cannot be weighed; its weight is estimated from dimensions, which is less precise and cannot be matched to a loose-stone report.
The right sequence
Buy the stone or bead loose. Have it tested and reported, with weight and dimensions. Then have it set, by a jeweller who understands the material (no heat near filled or oiled stones; no forcing of caps). Keep the report; note the setting on it or with it. If the stone is later removed and re-set, the report still describes it.
A stone bought already set can be tested in place with limits — magnification and refractometer on exposed facets, no SG — and the report states that it was examined mounted. For a full examination, a jeweller can unset it.
Metal testing
Gold purity is tested by hallmark, by acid or electronic testers (jewellers), or by XRF analysis (assay offices). The gem laboratory identifies the stone and may note the setting's appearance; it does not assay the metal. For a piece whose value lies in both, obtain a hallmark or assay for the metal and a report for the stone.
Frequently asked questions
Is panchdhatu a standard alloy?
No. The name describes a traditional idea; actual compositions vary widely and are not regulated or hallmarked.
Does the metal affect the stone's properties?
Not its physical or gemmological properties. Whether it affects anything else is a matter of tradition.
Can a laboratory test a stone in a closed-back setting?
Partially — magnification and RI from the crown, spectroscope, UV. Filling, dye and doublet joins may be hidden. The report states the limitation.
Should Rudraksha be capped in silver or gold?
Tradition and preference decide. The laboratory advises testing before capping, whichever metal is used.
Is plated jewellery a problem?
Only if sold as solid. Plating wears through and base metal beneath may react with skin. Hallmarks state what the metal is.