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Petrographic properties of some selected granite rocks in Ogun state, south western
Nigeria were used to predict rock strength index based on mineral constituent. In-situ test was
carried out on the rock samples using Schmidt hammer to evaluate its rebound hardness. The rock
sample mineralogy was determined in the laboratory using modal analysis. The rock samples were
prepared in accordance with International Standard for Rock Mechanism (ISRM) guidelines for
the determination of density, porosity, point load index, uniaxial compressive strength and equivalent
quartz content. The percentage mineral composition present ranged from 23.4 – 33 % microcline;
32.4 - 53.8 % quartz; 7.7 – 20.5 % biotite; and 20.9 – 31.3 % plagioclase feldspar. The average
density of granite samples ranged from 2.69 - 2.71 g/cm3 while their average porosity ranged from
2.33 - 3.93 % classified as low porosity. The average uniaxial compressive strength value ranged
from 52.5 - 172.5 MPa classified as averagely high strength, average point load index ranged from
1.95 - 2.94 MPa classified as averagely medium strength, while average tensile strength ranged
from 2.92 - 4.40 MPa which falls under high strength. The equivalent quartz content value ranged
from 55.96 – 79.65 %. The statistical mathematical model equation relating petrographic strength
of the rock constituent was developed. The equivalent quartz content for Jia Bao, Milatex, CNC, and
CCECC was satisfactory indicating high percentage of quartz in the mineral constituent of the
granite rock making it suitable for construction works.