Hi Billie, your pendant is lovely!
From what I can see in the photos, the opal displays a wonderful play of colour. The diamonds, assuming round brilliant-cut diamonds, set off the opal within a warm 18ct gold surround making it a lovely timeless pendant. Opals are very popular at auction, especially if they display the full spectrum of colours. The estimate given may be conservative, it would need to be seen first hand by a specialist for a very accurate valuation. Many thanks.