Weighing the facts: how size, shape, cut and carat weight affect opal value.

Large opal is very rare. This does not necessarily mean that big pieces cost more per-carat than smaller stones. In general, the largest and smallest stones will command the least per-carat. The most valuable opal size is that which is suitable for jewelry setting. As for shape, don't expect to find any faceted stones on the market. This is because faceting doesn't enhance opal's play-of-color and brilliance as with transparent gems, making the cutting style unnecessary. Typically opal is cut into either oval or freeform cabochon (dome) shapes.

When shopping for any gemstone, always think in terms of cost-per-carat rather than total cost. Since so many factors go into determining the cost-per-carat of each gemstone you look at (two one carat opals can have substantially different cost's-per-carat depending on brilliance, play-of-color, etc.) if you ask your jeweler the cost-per-carat of each stone, you will save yourself a lot of confusion.

Also beware of jewelry with a "total weight" price tag. You don't want to pay per-carat opal prices for gold settings.