The main RuneScape Wiki has an article on: Gold certificate

A Gold certificate is a certificate that is obtained by exchanging five gold ores with Giles at Draynor Village market or with Jinno at Zanaris. Gold ore certificates are stackable and each one can be exchanged back for five gold ores by talking to Giles.

Because gold ores are not stackable and players can only trade twelve items per trade screen, they are often exchanged for certificates so that they can be traded easier and faster.

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