Rock tumbling, also known as tumble finishing or rumbling, is a smoothing and polishing technique used to make glass-like rocks. The process is done inside special devices called rock tumblers, which consist of a barrel with grit inside and a motor.
Rock tumbling is a great hobby for everyone, and shaping rocks to a glass-like finish gives enormous satisfaction.
You can spend hours searching for and polishing different rocks and making a unique collection. Let’s look at rock tumbling in more detail and discover what rocks are good for tumbling.
The Process of Rock Tumbling
The process of tumbling rocks begins with finding the proper rocks to polish. In the next sections, we’ll discuss the characteristics of good tumbling rocks and the most suitable rocks for tumbling.
When you first find and collect the rocks, they will be rough, coarse, and ready for sanding.
You then place the rocks inside the tumbler, ensuring they have similar hardness. If one of them is harder, they will destroy the others in the process.
The tumbler usually consists of a barrel and a motor that spins it. The inside of the barrel contains the rocks, water and grit. The process always begins with rougher grit (around 60/90 grit).
The tumbling process takes several stages, and it can last for more than a month, depending on how hard the rocks are. With each new stage, finer grit is added. For example, the second stage starts with 150/220 grit.
Four or five stages may be required to polish the rocks, finishing with a polishing agent like cerium oxide (opticians’ rouge) and some cushioning material.
Characteristics of Good Tumbling Rocks
Good tumbling rocks have several important characteristics:
Hard and dense
Hardness plays an important role in determining what rocks are suitable for tumbling. A great reference point is the Mohs Hardness Scale, according to which rocks with 6-8 scale hardness are the most suitable for polishing.
Density is closely related to hardness, meaning dense rocks have minimum pores on a microscopic level. This results in finer and better-looking polish without grit marks on the rocks’ surface.
Fracture-free
Rocks with tiny visible fractures are not hard enough to withstand polishing and will most likely crack or produce a lot of dust. Fractures on rocks may also be covered in dirt, which will dissolve with polishing and do more harm than good.
Rocks with large fractures on the surface may even break into several pieces when tumbling, scratching every other rock in the barrel.
Properly sized (around 1 inch thick)
Size is important, as it will ensure that the rocks tumble inside the barrel easily and get polished faster. A 1-inch thick and 1-inch wide rock will tumble better than a larger, heavier rock.
Free of moisture and have vibrant colors
Another important characteristic of good tumbling rocks is that they are free of moisture, grease, or other contaminants. If they are not clean during tumbling, they might transfer these chemicals to other rocks, ruining the whole batch.
Lastly, the rocks should be colorful and vibrant. Otherwise, they wouldn’t be appealing, right? There are many different colored rocks to pick from, and we’ll cover the best rocks for tumbling in the following section.
The Best Rocks for Tumbling
Let’s start by looking at a quick table of all the best rocks for tumbling, followed by an explanation of each of their characteristics.
| Name | Characteristics | Color | Hardness |
| Agate | Most popular; Semi-translucent and easy to polish | A mix of red, yellow, brown, white, and pink | 7 |
| Quartz | The most common mineral with an ideal hardness to tumble and polish | Either clear, white, black, pink, orange, or yellow | 7 to 7.5 |
| Jasper | It’s considered a variety of quartz. It is opaque to semi-translucent | A mix of reddish, yellow, and brown | 6.5 to 7 |
| Obsidian | A glassy volcanic rock, slightly harder than regular glass. | Dominantly black, with red, brown, or golden hues present | 5 to 5.5 |
| Tiger’s eye | Metamorphic rock with opaque transparency | Varieties of brown and yellow | 7 |
| Aventurine | A variety of quartz with glistening properties. Easy to shape | Dominantly green | 6.5 to 7 |
Agate
The most popular rock for tumbling, agate has a spectrum of colors that goes extremely well with its translucent look.
You can polish it quite well, but it takes several stages to arrive at a glass-like finish due to its hardness.
The resulting polished rock is beautiful and gives out an internal glow.
Quartz
Quartz is one of the most common minerals in the Earth’s crust and has very high-value hardness. It is transparent, like agate, with some varieties being clear with a white fog tint.
Quartz can easily be polished, even though it’s a hard rock. The result is a smooth, glass-like surface rock with multiple color varieties.
Jasper
When you compare agate and jasper together, you can see they are very similar. But unlike agate, jasper is quite opaque. It is an aggregate of quartz with a variety of colors.
Jasper is a great rock to tumble, as it polishes quite similarly to agate.
Obsidian
Think of obsidian as a molten volcanic rock that cools down into a kind of black glass. Although it’s slightly harder than glass, it is considered a softer rock according to the Mohs hardness scale.
This makes it a good rock for tumbling; however, you shouldn’t mix it with much harder rocks as they might destroy it.
Tiger’s eye
A chatoyant rock, tiger’s eye, has multiple brown and yellow hues. It is also a derivative of quartz and has a very opaque surface and look.
Because it’s a rock with high hardness, you can polish it down to a mirror finish but with patience and consistency. That is when its chatoyancy effect will truly appear.
Aventurine
The last rock we’ve included in this article is considered another quartz-based gemstone. Because of the presence of fuchsite, Aventurine has a dominantly green hue color and an opaque look.
Aventurine has a lower hardness than ordinary quartz, but you can mix them in a rock tumbler without fear of one being ruined.
When properly polished, Aventurine produces a glittering effect when viewed from different angles.