The grand exchange is one of the easiest place to sell jewery, or as a matter of fact most items. Otherwise the best free-to-play NPC to sell them to would be the gem merchant in al-karid (near to the gnomish glider) you can sell anything to anyone anywhere but be careful theyre not ripping you off. Other than those, there is also a slightly more deserted place to sell gems which is to a shop behind the smaller bank of Falador.
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get any jewels and a chisel and with the right crafting level you can cut them!
no you can not sell your house on runescape.
Selling runescape gold in the real world is illegal. You can "sell" runescape gold in-game for other items on the gran exchange locatedin the north-west corner of Varrock.
At the Grand Exchange.
You can sell them to noobs.