Best Grits for Rock Tumbling: A Complete Guide

The Science and Magic of Rock Tumbling

Rock tumbling is a fascinating process that mimics the natural erosion found in riverbeds and along beaches, but at a vastly accelerated pace. Whether you are a beginner looking to polish your first batch of collected stones or an experienced lapidary artist refining your technique, understanding the grit sequence is the foundation of successful tumbling. The journey from a rough, jagged rock to a smooth, mirror-like gem is achieved through a carefully controlled series of abrasive stages. In this guide, we will explore the essential grits used in rock tumbling, the differences between abrasive materials, and the best practices to achieve that elusive perfect polish.

The 4-Stage Tumbling Sequence Explained

The standard rotary tumbling process is divided into four distinct stages, each serving a specific purpose. Skipping a stage or rushing through one will inevitably lead to subpar results.

Stage 1: Coarse Grinding (60/90 Grit)
This is the shaping stage. The primary goal of Stage 1 is to remove the sharp, rough edges of your stones and begin shaping them into pleasing, rounded forms. For this stage, a coarse 60/90 mesh silicon carbide grit is typically used. Because this stage does the heaviest lifting, it often takes the longest. Depending on the hardness of the rough rock—such as agates or jaspers—you may need to run this stage for several weeks, changing the grit and water every 7 to 10 days until the stones are properly shaped.

Stage 2: Medium Grinding (150/220 Grit)
Once your stones have been shaped and all deep pits or crevices have been ground away, it is time to move to the medium stage. Using 150/220 grit, this stage is designed to remove the deep scratches left behind by the coarse grit in Stage 1. It refines the surface of the stone, making it smoother to the touch. This stage usually takes about a week in a standard rotary tumbler.

Stage 3: Pre-Polishing (500/600 Grit)
The pre-polish stage prepares the stone for its final shine. Using a fine 500/600 grit (often aluminum oxide at this stage, though fine silicon carbide can also be used), this step removes the micro-scratches from the medium stage. After a week in pre-polish, your stones should feel incredibly smooth and may even begin to show a slight, dull luster when wet. Thorough cleaning between Stage 2 and Stage 3 is absolutely critical.

Stage 4: Final Polish
The polishing stage is where the magic happens. Instead of a traditional grit, this stage uses a microscopic polishing compound, such as aluminum oxide polish or cerium oxide. The polish does not grind the stone; rather, it burnishes the surface to a high gloss. This stage can take one to two weeks. The quality of your final polish depends entirely on how well you executed the previous three stages.

Silicon Carbide vs. Aluminum Oxide

When shopping for tumbling media, you will encounter two main types of abrasives: silicon carbide and aluminum oxide. Knowing when to use each is key to getting the best results.

Silicon Carbide (SiC)
Silicon carbide is extremely hard—rating a 9 on the Mohs hardness scale—and is characterized by its sharp, angular particles. When it breaks down during the tumbling process, it fractures into smaller, equally sharp pieces. This continuous self-sharpening action makes silicon carbide the ideal abrasive for grinding and shaping. It is universally used for the coarse (Stage 1) and medium (Stage 2) steps of the tumbling process.

Aluminum Oxide (Al2O3)
While also very hard, aluminum oxide behaves differently than silicon carbide. Its particles are blockier and tend to round off as they break down, rather than fracturing into sharp edges. This rounding effect is what you want in the later stages of tumbling because it prevents new scratches from forming on your nearly finished stones. Aluminum oxide is the standard choice for the pre-polish (Stage 3) and final polish (Stage 4) stages. Using silicon carbide for the final polish is not recommended, as it will continually leave micro-scratches on your stones.

How Much Grit Per Pound of Rock?

A common question among beginners is exactly how much grit to add to the tumbler barrel. The general rule of thumb is to use 2 tablespoons of grit per pound of rock.

For example, if you are using a standard 3-pound capacity rotary tumbler barrel, you should add about 6 tablespoons of grit. It is important to measure by the weight of the rocks, not the capacity of the barrel, though your barrel should always be loaded to about 2/3 to 3/4 full for optimal tumbling action. Using too little grit will increase the time needed for each stage, while using too much is simply a waste of material and money. The grit breaks down into a muddy slurry, which acts as the grinding medium. Too much grit can make the slurry too thick, inhibiting the tumbling action.

Common Tumbling Mistakes to Avoid

Even experienced rockhounds can occasionally face issues with their tumbling batches. Here are the most common pitfalls and how to avoid them:

• Inadequate Cleaning Between Stages: This is the number one cause of failed polishes. Cross-contamination occurs when even a single grain of coarse grit makes its way into the pre-polish or polish stage. It will gouge the stones, ruining weeks of work. Wash your rocks, the barrel, and the lid meticulously between every stage. Many people use a dedicated toothbrush to scrub crevices.
• Rushing the Coarse Stage: Stage 1 is the most critical. If a stone still has deep pits, cracks, or rough edges, it is not ready for Stage 2. Polish will not hide a bad shape; it will only make it shiny. Be patient and let Stage 1 run as long as necessary.
• Underfilling or Overfilling the Barrel: A barrel that is less than half full will cause rocks to crash into each other, leading to bruising and chipping. A barrel that is too full will not allow the rocks to cascade properly, halting the grinding process. Always aim for a 2/3 to 3/4 full barrel.

The Role of Ceramic Media as Filler

As your rocks progress through the stages, they lose mass. A barrel that was perfectly 3/4 full in Stage 1 might be only half full by Stage 3. To maintain the proper volume and ensure the stones tumble correctly without crashing, you must add filler. Ceramic media—either cylinders or spheres—is the best choice for this.

Ceramic media is non-abrasive and acts as a cushion, protecting fragile or soft stones from fracturing. It also helps carry the grit into small crevices. When purchasing ceramic media, note that it must be broken in. Running a new batch of ceramic media in a coarse grit for a day or two will remove its sharp manufacturing edges before you add it to a polishing batch.

Brand Recommendations and Buying Advice

When selecting your tumbling grits, consistency in particle size is paramount. Cheap, poorly graded grit can contain larger particles that will severely scratch your stones in the later stages. Brands like Lortone, Covington, and specialized lapidary suppliers offer high-quality, accurately graded abrasives.

If you plan to tumble frequently, buying grit in bulk (such as 5 or 10-pound bags) is significantly more cost-effective than buying small kits. Always store your grit in sealed, clearly labeled, waterproof containers to prevent contamination and moisture absorption.