Black Opal vs. Boulder Opal

Black opals are typically mined in the Lightning Ridge region in New South Wales. The black opal is mined in shafts and is widely regarded as the most valuable kind of opal. The Boulder opal was first discovered in Quilpie, Western Queensland. This kind of opal is second only to the black opal in terms of value.

Body Tone

Black Opal - The body tone of a black opal can range anywhere from dark grey to black. Its dark body tone allows it to display colours more vibrantly than other kinds of opals. Typically, a black opal will integrate a natural layer of potch onto its back.

Boulder Opal - Although boulder opals also have a dark body tone, they derive this from a layer of ironstone on their backs. As a result, the boulder opal is sometimes called a 'natural doublet'. The ironstone layer allows the stone to also display colours vibrantly.

Colour & Pattern

Black Opal - A black opal will expertly display all the colours in the visible spectrum, in an array of different patterns that include floral, ribbon, harlequin, straw, Chinese writing and rolling flash.

Boulder Opal - This kind of stone also displays all the colours of the rainbow but the patterns it does so in will vary.

Stone Shape

Black Opal - Most black opal stones are oval in shape, but it is not uncommon to find one cut ‘freeform’ to maximize the stone weight.

Boulder Opal - Since boulder opals contain only thin layers of opal, they often cannot be shaped and cut like other opals. These stones generally have irregular shapes in order to maximize their carat weights.

Strength

Black Opal - The black opal is identified as having a hardness of 5.5 to 6.5 on Moh's Hardness scale. A diamond has a hardness of 10. The black opal can broadly be said to be as hard as glass.

Boulder Opal - The ironstone layer on a boulder opal gives a boulder opal more strength and protection. They are likely to survive greater impacts and pressure than black opals.

Rarity

Black Opal - The black opal is the rarest opal in the market, and is mined within a 70km radius in Lightning Ridge. Black opal of the highest quality is only found in Australia.

Boulder Opal - The region in which boulder opal is mined is far larger than that of black opal, making it more abundant in the market. This area stretches more than 500 kilometres in Western Queensland.

Price

Black Opal - Since black opals are rare, they will fetch good prices in the jewellery market. The finest black opal may even fetch a price that is seven times greater than that of a boulder opal.

Boulder Opal - Relative to black opals, the boulder opal is heavily under-priced. On average, a boulder opal will fetch a price that is a third of an equivalent black opal.