Your browser is not supported anymore.

We suggest changing from Internet Explorer to another option. The Internet Explorer browser is no longer supported by Microsoft. Please install or upgrade one of the browsers below.

How abrasive or fractured is your rock

posted by the Epiroc Exploration & OreBody Solutions team

November 16, 2021

For optimal drilling performance, you need a drill bit that is designed to handle a variety of variables that are particular to your drilling project. Choosing the right core bit is one of the most important decisions you will make, and the type of ground is an important consideration. The hardness of the ground can be determined by performing a scratch test with an etcher kit, but this blog will explore the other factors that affect the drillability of rock, such as competency.


Rocks are classified based on chemistry and structure.  The hardness is classified on a relative scale referred to as Mohs hardness scale. To choose the right matrix for the hardness of the rock, you refer to a matrix selection chart that gives you options for each category of hardness. Adding to the complexity is the fact that a specific rock type can change drastically within the same borehole. This known as variability and manufacturers have designed core bits that are excellent at dealing with ground variability, such as the Viking line.


In addition to hardness and variability, you will also need to consider other characteristics that affect how drillable your rock will be. Other factors that are most important are grain size, weathering and fracturing. The strength of the rock is reduced when there is weathering.

So how do you know where your rock lies when it comes to abrasiveness and competency? Let’s look at some examples of rock and suggested core bits.


Very abrasive badly fractured rock

Common types of badly fractured and very abrasive ground are sandstone, shale, limestone and conglomerate, especially with a high silica content.

Diamond Tools Broken Ground Fordia 2021 Ground Abrasiveness

Moderately fractured and/or abrasive rock

Weathered granite and gneiss, dolomite, tuff and schist are all common types of moderately fractured rock, with or without abrasiveness. 

Moderately abrasive rock

Common examples of moderately abrasive rock include basalt, gabbro, peridotite, diorite and gneiss

Competent, moderately abrasive rock

Rock formations that are competent and moderately abrasive are metabasalt, amphibolite, diorite, granite and diabase.

Hard, competent and slightly abrasive rock

Granite and pegmatite are examples of hard, competent rock with slight abrasiveness.

Very hard, competent and slightly abrasive rock

Metamorphozed granite and quartz rich gneiss are examples of rock that is very hard and competent, with slight abrasiveness.

Extremely hard, very competent rock

Chert, jasperite, quartzite and metamorphosed volcanics are all common rock formations that are extremely hard and very competent.

Simple scratch tests can guide you in determining the hardness of rock but deciding how fractured your rock formation is can be trickier. Hopefully, the illustrations above can help you decide. Our goal is to improve drilling performance and accelerate the transformation of our industry. If you need any advice on rock formation or anything else, our technical support team can help. Don’t hesitate to reach out to them.