Gemstone identification

So I have an antique cravat pin from Sweden, from the 1910s; 1918, I believe. I got it from a big antique store on an auction site at a pretty reasonable price. According to them, the pin itself is 18k gold, which seems reasonable, as it is rigid and keeps a nice point, but is bendable, and has a typical pale-ish golden colour - of course, for all I know it could be some imitation metal and I would be none the wiser. It matters little. Around the setting at the head of the pin are small, small pearls (1-2mm diameter), that appear genuine, with irregularities in luster, although they are hard to investigate.



Now, in the middle, is a decent-sized gemstone, about 7-8mm, in a typical oval shape. I have been trying to figure out what gemstone it is. It shifts a lot but at its heart it has a pale, somewhat water-like blue tint - it appears a glass-like clear when looking straight through it (the gemstone setting has no back). It has no scratches and no sign of wearing.



I have deduced that it ought to be either a pale topaz, or a pale aquamarine - both are very hard and not unreasonably uncommon.



Is there any way to differentiate them to a layman? It is mere idle curiosity from my side, but it would be cool to know. I have a 20x loupe, but little else in the way of equipment.

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