The purchase, as well as sale values, are converted into INR per the sale date. Capital gains are calculated in INR, and the subsequent tax is calculated based on the capital gains. Technically, there is no tax on INR depreciation.
Example: You bought 1 share of Amazon at 2,000 USD on 1st January 2020 and sold at 3,000 USD on 1st December 2020. USD to INR rates are 70 INR and 75 INR respectively
Capital gain = Qty * (Selling price - Buying price) * FX as on Selling date= 1* (3,000 USD - 2,000 USD) * 75 = 75,000 USD
On conversion of USD to INR, Rule 115(1) states that the rate of exchange shall be the telegraphic transfer buying rate of the last day of the month immediately preceding the month in which the shares are sold.