Sri Navaratna - Nine Gems:
The traditional setting arrangement for these 9 gems is shown in the below illustrations (pendant and ring). Ruby for Sun is always in the center, surrounded (clock wise from the top) by diamond, natural pearl, red coral, hessonite, blue sapphire, cat's eye, yellow sapphire, and emerald.
This is the same placement as the Nava-graha Yantra.
Traditionally no gem other than ruby (for the Sun) is set in the center of a nine gems arrangement. To do so is believed to go against the flow of nature.

Sri Navaratna Ring

The Sun is the center of the solar system and therefore the Sun's gem is always positioned in the heart of a Navaratna Talisman.