Hi Sir,
Interesting question - My thinking says -
It cannot be taxed under Income Tax Act - It is not an "Income", provided he can prove that he acquired them on digging otherwise Sec 69A can be invoked-
69A. Where in any financial year the assessee is found to be the owner of any money, bullion, jewellery or other valuable article and such money, bullion, jewellery or valuable article is not recorded in the books of account, if any, maintained by him for any source of income, and the assessee offers no explanation about the nature and source of acquisition of the money, bullion, jewellery or other valuable article, or the explanation offered by him is not, in the opinion of the [Assessing] Officer, satisfactory, the money and the value of the bullion, jewellery or other valuable article may be deemed to be the income of the assessee for such financial year.]
Wealth Tax shall be levid on these two items.