PURIX
AI characterWhat is a touchstone?
A touchstone is a small, dark stone tablet, such as slate or lydite, used to test precious metal alloys. Its surface is fine-grained and leaves a visible streak from soft metals such as gold. The stone must be free of cracks and must not react with acids.1
The method is very old: the touchstone was used in the Harappa period of the Indus Valley (around 2600–1900 BCE) and in Ancient Greece.1
How is the test done?
The test uses an acid test kit. Each needle in the kit has a gold sample of known karat at its tip, and the karat is written on the needle. Yellow gold needles are used for yellow gold, and white gold needles for white gold.2
How is the result read?
Acid does not dissolve streaks of a karat higher than it is strong enough for. If the unknown streak remains, the piece is higher than the karat that acid tests; the test is then repeated just below the same streaks with the acid for the next karat up.2
| Acid applied | What do you see on the stone? | What does it mean? |
|---|---|---|
| 10 karat acid | The 10 karat streak dissolved, the unknown streak remains | The piece is higher than 10 karat |
| 14 karat acid | The 10 and 14 karat streaks dissolved | The piece is 14 karat or slightly above |
Jewellers in the Grand Bazaar describe the same logic: acids of different concentrations, set for 14, 18 and 22 karat, are used.3 For 14 karat and below, nitric acid of specific concentrations is used; for higher karats, aqua regia, a mixture of one part nitric acid and three parts hydrochloric acid, is used.2
If you doubt the piece is gold
First a small notch is made in an inconspicuous spot on the piece, and a drop of 18 karat acid is placed in the notch. A foaming, green reaction shows that the piece is a metal that is not gold; in that case the karat test is not carried out.2
What should you watch out for?
Because the test is done with acid, gloves and safety glasses are worn and the work is done in a well-ventilated place.2 In the past, jewellers tried to judge gold by biting it, by its softness or by the sound it made when dropped; today the acid test is used.3
FAQ
What kind of stone is a touchstone?
A dark, fine-grained stone such as slate or lydite. It must be free of cracks and must not react with acids.
Which acid is used in the touchstone test?
For low karats, nitric acid of specific concentrations is used; for high karats, aqua regia, a mixture of nitric and hydrochloric acid.
What is a karat needle for?
It has a gold sample of known karat at its tip. It is used to compare the streak drawn on the stone with the streak of the unknown piece.
Does a touchstone damage jewellery?
Because the piece is rubbed on the stone, a small amount is taken off; GIA describes the method as an old technique that does relatively little damage to jewellery and gives quick results.
Karat isn’t guessed. It’s measured.
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