Rock polishing

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Snowflake obsidian after tumble polishing

Rock polishing is a lapidary process whereby rough stone is polished and smoothed by hand or using simple machines to produce attractive stones. The most common means is tumble polishing, but a vibratory finishing process can be used instead.

To effectively polish rocks in a tumbler the rocks must all be of the same approximate hardness. Rocks are placed into a tumbling barrel with varying degrees of abrasive grit and often with water. Depending on the hardness of the rocks, each step in the process can continue over a number of days of continuous operation of the tumbling barrel.

Polished rocks start out as something called a "tumbling rough", an untreated stone. A tumbling rough is placed into a tumbling barrel with abrasive grit and water. This initial procedure is called the coarse grind. The purpose of the coarse grind is to wear down any sharp or uneven edges on the rock's surface, eventually producing smooth round stones.

The second stage of rock polishing is the fine grind. This process is the same as the coarse grind, differing only in the use of a finer grit, and that the stones are no longer shaped but are beginning to be polished. At the end of this step the rocks would appear shiny when wet, but matte when dry.

The pre-polish step uses a mixture of very fine grit and special plastic tumbling pellets. At the end of the process, the stones have a slight luster when dry.

The final two steps are the polish and in some cases the burnishing. In the polishing step, rock polish is added in place of grit as well as the plastic tumbling pellets. After further tumbling, the rocks should now have a shiny look when dry. If this is not the case and the rocks appear to have a film on them, a burnishing step may be necessary.

In burnishing, the rocks are tumbled with only the plastic pellets and the addition of an oil-free non-abrasive soap.