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I have a gold ring with I assume a ruby cabochon and diamonds.
The ring is marked 90%, no other hallmarks, tests positiv for 18K gold and weighs 13.5g.
Does anybody know the origin of this ring? Which countries use or used 90% to mark gold? Thank you!

It should be 22 karats (carats being the weight of precious stones)if it`s 90% gold purity (or more precisely 87 per cent gold for 22K).
So, test it again but sometimes it`s lower than declared if not made in well legislated countries. Have seen jewelry made in the Far East stamped with percentage, unlike Europe and North America.
Is it a natural ruby? Hard to say; use magnifying glass and search for inclusions - ``needles``,``silk``. Furthermore, not every red gemstone is a ruby and even natural pale red corundum is debatable to classify as a ruby.