Classification of rocks and building stones in Kenya.

The word Stones is  what we call natural  rocks after quarrying them out  of the earth’s crust .According to the  Building Stone Institute  of  America(BSI) and  others like Arthur Lyons rocks can  be  put  in three  geologic  categories :(1)Sedimentary rocks (2)Igneous rocks and (3)Metamorphic rocks.Below  we shine alight  on a few  details relating to each  category.

1.Sedimentary rocks-are  formed  by  weathering and  erosion of  older rocks.This  action by water,ice and wind breaks  the  rocks  into progressively  smaller  pieces  and  are  carried  by  rivers   to  oceans  and  other water  bodies.These  deposits  are  laid down  as  sedimentary beds of mud or sand,which build up layers,become  compressed and are in the end   cemented together by  minerals  like calcium carbonate ,quartz,iron oxide or by  dolomite found in underground water.

The bedding planes   that  are natural  in sedimentary  rock  are  put to use  when quarrying the stone  and  to  obtain  maximum durability  in masonry  building.Quarried  sedimentary  rock  contain quarry sap   allowing  easy  carving and work on them;however after exposure  to the  open air,they become hard like nails  to  work with. Example include Sandstone, limestone, and  dolomite

2.Igneous rocks-Formed when magma (hot liquid rock within the earth) cools. Depending  on whether  solidification happens slowly  within the earths crust or rapidly at the surface the  Igneous  rocks  can be defined  as  Plutonic or Volcanic respectively. Plutonic rock’s slow  cooling  allowed  large  crystals  formation which   can  be seen in granite. Volcanic rocks  like basalt and pumice  are  fine grained.Examples are Granite, Syenite, Diorite, Gabbro, Andesite and Basalt.

3.Metamorphic stones-formed from sedimentary or igneous rocks by action of heat and pressure.To  put it  differently,they  are formed  by  the crystallization of  older rocks  by the action of  intense  heat and  or pressure within the earths’s crust. Clay  is changed into  slate,limestone to  marble  and  sandstone  to  quartizite.dscn0082

A building under construction using Nairobi building stones

Building stones  in Kenya

Having dealt with the classification of  Rocks,allow me  shift  to something that is  equally  interesting, and that is  building stones  used in Kenya and  beyond.They  are quite a number and include  the following:

Granite stones:

  • They  are  dense and  hard impermeable to water and resistant to impact damage.Granite  can be sawn,rough punched,picked ,fined  tooled,honed ,polished  or flamed to give them a perfect finish.They are  either  fine ,medium  or  coarse grained  igneous  rocks composed  of quartz, feldspar, and  Colors range from pink, red, gray, blue, green, tan, brown, black and every color and shade between.
  • Because they  are expensive  granite is  use most often as  cladding  material(40mm externally or 20mm internally).Or  it   can be cast  directly  onto concrete  cladding  units.
  • Granite  can also be used for flooring,hard landscaping  pavements etc.Finally  polished  granite  can be used  as kitchen  counter tops.

Cast basalt:

  • Its  a fine grained stone   that  is  hard  and  can melt at  2400°C.This  can be cast into  tile units which are  deep  steel grey  in color.
  • Larger cast units  for worktops ,either hone  or  polished can be cut to size.


  • This  is  limestone rocks that  have been changed by the action of intense heat and pressure in the earth’s crust.
  • The calcium carbonate or Serpentine is recrystallized into a mosaic of  near  equal-sized  calcite crystals
  • Calcites  is  white ,so  a pure  marble is white and  translucent.The colors and veins   in a typical marbles are due to impurities within the original  limestone.These range from  red,pink,brown green ,beige,cream,white,grey  and  black.
  • Its  attached  by acids ,therefore  should be honed  not polished for external uses.
  • They  are  hard and dense but  fissures and veins may require  filling
  • Can be used  for cladding and  fixing clamps and  hooks  used need be in stainless steel phosphor  bronze  or copper
  • Floor  slabs should be a minimum of  30mm,laid on minimum 25 mm bed.
  • Marble wall and bathroom floor tiles should be  between 7mm to  10  mm in thickness


  • They are fine to medium grained sedimentary of rock having at least 60% free Silica. Colors range from grey, yellow to brown.
  • They  can be  sawn,split faced,clean rubbed or tooled to  give them an attractive finish.
  • To use them for  cladding-75mm to 100mm thickness is used  and  fixed with cramps and corbels
  • dscn0083
A finished building constructed with natural stones


  • It’s a  sedimentary  rock composed  of  classic  and  sand-sized grains  of  calcites, fossils  or  shell .
  • Composed mainly of  calcium carbonate either crystallized  from solution as calcite or formed from accumulation of   fossilized shell deposits  by  various  sea  organism.
  • Colors range from light grey, tan to light brown.
  • They  can be  fine-rubbed,fine dragged,split faced or  tooled to desired  finish.
  • Externally limestone  must not be mixed with  or  located  above sandstone, because  they may  negatively affect  the sandstone.


  • Its fine grained metamorphic sandstone.
  • Colors range from shades  of  blue  grey, green, and  red.


  • Formed  from fine grained sand free clay sediments
  • They are  fine grained  rocks  which  split easily  along  its  cleavage
  • As a result slate  can be  split into thin sections(4-10mm) for  roofing slates
  • Slate can be  sawn, sanded,fine rubbed,honed,polished,flamed or  bush hammered.
  • Spain  is  world’s largest producer  of  slate
  • Its  strong, acid  and  frost  resistant,lasting up to  400 years as  roofing material.
  • The minimum recommended  pitch for slate roofing is 20°  under  sheltered exposures and  22.5° under  severe  exposure.
  • Fixing nails  should be of  copper or  aluminium
  • It  can be used  for flooring ,cladding,copings,cills and  stair  threads
  • Colors  range  from grey, purple ,red  and brown.


  • These are a foliated metamorphic quartz –feldspar containing rock.
  • They often have mica or chlorite. Schist split readily along planes of foliation.


  • This  is  sandstone that has metamorphosed    because  of intense heat and pressure.
  • The grains  of  sandstone  are  recrystallized into a matrix  of  of quartz  to form a  a hard to wear  stone used  as a flooring material.
  • It has  mica which allows the stone  to be split along smooth  cleavage  planes.
  • Exported by  Norway and  South Africa.


  • A naturally  occurring  gypsum or calcium sulfate
  • Used in making  carved  monuments  and  ornaments
  • In pure form its  white and translucent
  • Traces  of  iron oxide  may give it  light brown,orange  or  red color


Copyright © 2017 East Africa Little Hardware Ltd

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