The BIS hallmark consists of 5 parts.
1) BIS mark
2) Purity - Purity is defined by a number - 958 for 23 carat, 916 for 22 carat, 875 for 21 carat, 750 for 18 carat, 585 for 14 carat and 375 for 9 carat. The number in simple terms defines the percentage purity of gold in the jewellery item. 916 for example means 91.6% purity. Similarly the carat is also derived with this percentage - 24 * 91.6 = 22. The remaining percentage, 8.4% in this example is Copper or Zinc.
3) Assaying and Hallmarking Centres mark - The hallmarking on a jewellery item is actually done at the hallmarking centres. If you are a regular buyer, you will notice your jewellery being hallmarked at the time of buying instead of that being done in advance. This is usually the case for newly arrived jewellery pieces.
4) Year of Marking - The year making is identified by a letter which is decided by BIS. For example, A stands for 2000.
5) Jewellers Identification Mark - The final and fifth part if the identification mark of the jeweller.
Below is a sample hallmarked jewellery from a jewellery store in Hyderabad.