Should I Tumble Rocks with Sharp Edges? A Rock Tumbling Guide

If you're new to rock tumbling, you might look at your jagged, rough-edged stones and wonder, "Is it safe to put these sharp rocks into my tumbler?" It's a very common question, and the answer is likely the opposite of what you might expect!

Tumbling Sharp Rocks: Why Jagged Edges are a Good Thing

The primary purpose of the coarse grit stage (Stage 1) in rotary tumbling is shaping. This process requires friction and impact to grind down the rough material.

When you start with a rock that has sharp edges and corners, you are giving the coarse grit a perfect, concentrated point of attack. The tumbling action focuses the wear on these highest, most vulnerable points, quickly chipping and grinding them away.

In short: You do not need to avoid tumbling rocks with sharp edges or corners—in fact, they are ideal for Stage 1! Starting with sharp material is the fastest way to get to the desired smooth, rounded shape.

Understanding the Dangers: What NOT to Put in Your Tumbler

While sharp edges are fine, there are other characteristics you should definitely avoid as they can ruin your batch or damage your equipment. The danger isn't the pointiness; it’s the integrity of the rock itself.

1. Avoid Rocks with Deep Pits and Cracks

This is the number one reason for a poor polish, and it’s a much bigger concern than sharp edges.

The Problem: Deep pits, cracks, or holes (known as vugs) can trap the coarser grit from Stage 1 or 2.

The Disaster: When you move to the finer stages (like pre-polish and polish), the trapped coarse grit leaches out and contaminates the fine slurry. This contamination will scratch the surfaces of all your rocks, completely ruining the final polish.

The Solution: Only tumble stones that have solid, smooth surfaces, or be prepared to run Stage 1 until those imperfections are entirely ground away (which can take weeks or months).

2. Avoid Mixing Rock Hardness

You should only tumble rocks of similar hardness (Mohs scale) together.

The Problem: If you mix hard quartz (Mohs 7) with softer materials like obsidian (Mohs 5-6) or softer decorative stones, the hard rocks will act like grinding media, aggressively damaging and pitting the softer rocks.

The Result: Your softer stones will be ruined, and the slurry will be inconsistent, leading to a poor finish on everything.

The Solution: Sort your rocks by hardness. For beginners, stick to tumbling only agates and jaspers together, as they are all Mohs 7.

3. Avoid Porous or Fractured Material

Rocks that are highly porous (like sandstone or chalk) or highly fractured (with visible fault lines) are not good tumbling candidates.

The Problem: Porous rocks will absorb the slurry and never take a good shine. Fractured rocks will inevitably break apart into tiny pieces during the process, wasting your tumbling grit and potentially unbalancing the tumbler.

The Solution: If you’re collecting your own rocks, perform a quick water test. Rocks that stay wet and do not absorb water quickly are usually good candidates.

The Rock Tumbling Process: How Sharpness Disappears

The tumbling process is designed to eliminate the sharp edges you start with:

Rotary Action: The barrel lifts the rock mass, which then cascades and tumbles back down.

Grit Grinding: The coarse silicon carbide grit acts as a powerful abrasive between the rocks.

Shaping: Over the course of the first few weeks, the grinding action relentlessly rounds all the sharp edges and corners until the entire stone is smooth and ready for the next stage.

In conclusion, embrace the sharp, rough stones for your first batch. They are exactly what your rock tumbler is designed to handle! Just focus your attention on avoiding pits, cracks, and hardness mismatches to ensure a successful, glossy final product.