My great grandfather recently passed away and has left some jewellery and cash for me. There is no will or any other written documentory evidence for the same. These are not mentioned in his return too. What to do now ? How to show in my return now and what will be tax implications if any. Thanks

Taxes on inheritance are paid by the estate, not the heir. Generally there is an exemption of $2 mil before taxes are incurred. If the estate wasn't probated, no worries.