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Although you do not have a credit card, but you can still buy the gems by using gift cards or vouchers. But it is quite complicated because you must have an email account from United States (US).
If you already have a US email account, you can buy it in the following way:
a. Open your account CoC
b. Open the "Shop", then select the "Treasure"
c. Choose the amount you want to purchase gems
(Pile = 500, bag = 1200, sack = 2500, box = 6500, chest = 14000)
d. For payment method, select the method redeem / exchange.
e. You must enter a voucher code or gift card. After that, select the store.
f. Enter your Google password
This method is quite risky, but it is quite easy. You simply transfer the money to the service provider charging gems. Then, you simply provide your email address and password you CoC account to the bank.
You can directly use carrier billing to buy gems. However, make sure you change the payment method.