F-Numbers, Microns and Abrasives – What’s It All About?

Rock tumbling is a hobby that blends creativity, geology, and a bit of science. Yet for many beginners—and even seasoned hobbyists—the terminology surrounding abrasives can feel confusing. What do F-numbers mean? How do microns fit in? And why does any of it matter when you're polishing stones?

This guide breaks down the essentials so you can choose the right rock tumbling grit, improve your tumbling results, and understand the science behind the shine.

What Are Abrasives and Why Do They Matter in Rock Tumbling?

Abrasives are the heart of the rock-tumbling process. Every polish, shine, and smooth surface you achieve comes from the careful use of abrasive particles grinding down rough rock into a smooth, reflective surface.

Different grit sizes shape the way your rocks progress through the tumbling stages:

  • Coarse grits shape and round rocks
  • Medium grits remove scratches and refine the surface
  • Fine grits prepare for polishing
  • Polishes give the final mirror-like finish

To understand grit size, you’ll often see two measurement systems: F-numbers and micron ratings.

What Are F-Numbers? The Grit Classification You See on Abrasive Packaging

F-numbers come from the FEPA (Federation of European Producers of Abrasives) grading system. They are commonly written as f80, f220, f600, etc.

How F-Numbers Work

Lower F-numbers → larger abrasive particles → more aggressive cutting

Higher F-numbers → smaller particles → smoother finishes

For example:

F80 ≈ coarse grit used for shaping hard stones

F220 ≈ medium grit for scratch removal

F600–F1200 ≈ pre-polish levels

Why F-Numbers Matter to Rock Tumblers

FEPA grading is known for consistency. This means each grit stage behaves predictably, giving you better control and fewer surprises during your tumbling cycle.

What Are Microns? A More Precise Way to Measure Abrasive Size

While F-numbers describe grit in a standardised range, micron measurements describe the actual average particle size. A micron (µm) is one-millionth of a meter.

Typical micron equivalents in rock tumbling:

  • Coarse (60/90 grit) → ~250–150 microns
  • Medium (120/220 grit) → ~125–70 microns
  • Fine (400–600 grit) → ~30 microns
  • Pre-polish (1000–1200 grit) → 5–15 microns
  • Polish (aluminum oxide, cerium oxide, etc.) → ~0.3–1 micron

Why Microns Are Important

Micron ratings tell you exactly how big the particles are, making it easier to compare abrasive types (for example, aluminium oxide vs. silicon carbide).

How F-Numbers and Microns Compare: Understanding the Relationship

Although both systems measure abrasive size, they don’t convert perfectly. An F-number represents a range of grain sizes, while microns represent an average particle size.

However, the two measurements generally line up like this:

Tumbling Stage Common FEPA Grade Approx. Microns Purpose
Coarse F80–F120 250–150 µm Shaping, rounding
Medium F220–F320 70–40 µm Scratch removal
Fine F500–F800 30–15 µm Smoothing before pre-polish
Pre-Polish F1000–F1200 10–5 µm Preparing for final polish
Polish N/A 1–0.3 µm Final shine

 

Understanding these helps you choose the right abrasives and know when to step up to the next stage.

Which Abrasive Materials Are Best for Rock Tumbling?

Different abrasive minerals have different hardness levels and properties. The three most commonly used in rock tumbling are:

1. Silicon Carbide (SiC)

Best for: Coarse, medium, fine grinding

  • Extremely hard and sharp
  • Ideal for shaping and smoothing harder stones
  • Breaks down slowly into smaller grits, improving efficiency

2. Aluminum Oxide (Al₂O₃)

Best for: Pre-polish and polish

  • Softer and more forgiving
  • Excellent for final stages
  • Produces a smoother pre-polish than SiC

3. Cerium Oxide, Tin Oxide

  • Best for: Specialty polishing
  • High-end polishes for glassy stones (agate, obsidian, jasper)

How Grit Size Affects Your Tumbling Results

Choosing the right grit progression is one of the most critical steps in successful tumbling.

A good grit sequence prevents:

  • Scratches from showing up in later stages
  • Incomplete polishing
  • Wasting time on ineffective cycles

A typical 4-stage sequence

Coarse (60/90 or F80) – shape the stones

Medium (120/220 or F220) – remove scratches

Fine (400–600 or F400–F600) – smooth surface

Polish (0.3–1 µm) – achieve shine

Adding a pre-polish (F1000–F1200) can dramatically improve the final finish.

How to Choose the Right Abrasive for Your Rock Tumbling Style
For beginners:

Use standard SiC grits and either cerium oxide or aluminium oxide polish. They’re affordable and effective.

For soft stones:

Use aluminium oxide earlier in the process to avoid bruising or excessive scratching.

F-numbers and microns might seem technical at first, but they are the keys to mastering rock tumbling. Knowing how abrasive sizes work—and how they translate across systems—gives you the precision needed to achieve consistently stunning results.

Whether you're shaping rough rocks or chasing that perfect mirror-polish, understanding abrasives puts you in control of the entire tumbling journey.

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