minerals "minerals" means minerals and mineral substances (other than mineral oils), and includes precious metals, precious stones and non-precious minerals, but does not include common clay (other than kaolin or bentonite), sand, sand-stone or any sodium compound or any other similar common mineral substance obtainable without underground mining operations and not containing any precious metal or precious stones in economically workable quantities; ACT 53 - INCOME TAX ACT 1967 18. Interpretation of Part III.