- The practice of using dark stones to test precious metals dates back to ancient times, and while obsidian can serve this purpose, it offers both advantages and challenges as a testing surface. This volcanic glass, with its smooth dark surface, provides an excellent background contrast for observing the characteristics of metallic streaks left by gold and other precious metals.
- The testing process using obsidian is relatively straightforward but requires careful attention. When a piece of gold is drawn across the obsidian’s surface, it leaves a visible streak that can be analyzed. This streak is then treated with nitric acid, which creates a revealing chemical reaction. Pure gold will remain unchanged by the acid, while lower-karat gold alloys will show varying degrees of dissolution. Items that aren’t gold at all will either completely dissolve or react strongly with the acid, quickly revealing their true nature.
- However, obsidian’s natural properties present certain limitations for its use as a testing stone. Its glassy structure, while providing a smooth testing surface, is more brittle than traditional testing stones like basalt or lydite. The very smoothness that makes obsidian attractive can sometimes work against it, as it may not create enough friction to produce an adequate streak for testing. Additionally, the glass-like composition of obsidian makes it more susceptible to damage from the acids used in testing compared to more traditional materials.
- For these practical reasons, professional jewelers and assayers typically opt for traditional black basalt or lydite touchstones rather than obsidian. These conventional materials offer greater durability and reliability in day-to-day testing situations. Nevertheless, obsidian’s historical use and its ability to serve as an improvisational testing stone make it an interesting alternative when traditional materials aren’t available.
