MadSci Network: Earth Sciences

Re: What are the uses for schist and conglomerate?

Date: Sun Jan 6 16:00:29 2002
Posted By: David Scarboro, Faculty, Earth Sciences, The Open University
Area of science: Earth Sciences
ID: 1008708745.Es

Dear Jennifer,

The uses to which rocks can be put depend on their physical properties and 
on the minerals they contain.  As I am sure you have discovered, schist 
and conglomerate are two quite different types of rock.  However, what 
they may have in common is that neither one is especially useful.  For 
example, neither schist nor conglomerate is an ore containing desirable 
elements such as iron or gold, so neither is quarried on a large scale.

This does not mean that they have no uses, however.  Schist is a 
metamorphic rock, which means that it was formed by the transformation of 
other types of rock under conditions of great heat and pressure deep in 
the Earth’s crust.  Metamorphic rocks come in a sequence of what are 
called metamorphic grades.  Rocks of low metamorphic grade, such as slate, 
are rocks in which the process of metamorphism has been halted at a 
relatively early stage; and rocks of high metamorphic grade, such as 
gneiss, have been subjected to extensive metamorphism at very high 
pressures and at temperatures just short of being high enough to cause 
them to melt.  Schist is intermediate between slate and gneiss, and is 
toward the higher end of metamorphic grade (closer to gneiss than to 

Gneiss is widely used as an ornamental building stone because it takes a 
fine polish and can be quite beautiful.  All metamorphic rocks show an 
alignment of their crystals because the high pressure in which they were 
formed tends to cause crystals to grow in the same alignment.  With gneiss 
this alignment of the crystals can come out when polished as a lovely 
pattern of banding of the different minerals (such as pink feldspars and 
glassy quartz).  Schist contains more mica than gneiss does, and since 
mica is a flaky mineral it is hard to work with.  Schist would be more 
difficult to polish than gneiss, and would tend to stand up less well to 
weathering if placed, say, on the outside of a building.  But, still, you 
might be able to find examples of schist used as a polished ornamental 
stone.  Where I live, in the United Kingdom, one sees gneiss quite 
frequently as a polished ornamental stone, but I don’t recall ever seeing 
a schist used in this way.

Another use for schist is that it is a source of the semi-precious 
gemstone called garnet.  Garnet is a mineral that is nearly always found 
only in schists, and there are many schists throughout the world in which 
large crystals of garnet can be found.  In the United Kingdom, for 
example, schists from Scotland are well known for their beautiful garnets.

Conglomerate is a sedimentary rock, formed by the transportation and 
deposition of eroded fragments of other rocks by the action of running 
water, as in rivers and along shorelines.  Conglomerate typically contains 
a wide range of sizes of “grains”, ranging from pebbles down to very fine 
material.  Because of this range of grain sizes conglomerates probably 
don’t make very good building material – they would lack the strength to 
support heavy structures and would be vulnerable to weathering.  But, like 
schist, they can make interesting and beautiful polished ornamental 
stone.  I have even seen unpolished conglomerates used as ornamental 
stone, always inside of buildings.

If you want to see examples of rocks used as ornamental stones in 
buildings, you can usually see plenty of examples in any town or city 
centre or in shopping malls, where the architects want to create beautiful 
and impressive surroundings.  Most of the rocks you will see will be 
crystalline igneous rocks such as granite and basalt, or high-grade 
metamorphic rocks such as gneiss.  You can tell them apart because the 
igneous rocks will be a mass of crystals without showing flowing or 
banding, whereas the metamorphic rocks will usually show flow patterns and 

I hope this answers your question.

Best wishes,

David Scarboro

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