"OLD IS GOLD".It's an unique collection.But as far as my knowledge goes diamonds aren't yellow in color.Your gemstone is called "Yellow Saphhire".
Diamonds come in every color in the rainbow including yellow but that stone does not appear to be a diamond. It could be a saphire but it also could be a pale Citrine or Yellow Imperial Topaz. One way to have some idea is to know what the metal is the stone is set in. A friend who is an antiques dealer pointed this out too me. If the stone is really precious, it should be set in gold and more likely a higher carot of gold.
Here is a link to a page that shows all the colors of Diamonds, they say there have been 300 different colors identified to date.
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