Untreated rough of precious chocolate opal with nice play-of-colour visible in several places when the light catches it just right, mainly flashes of bright green and red. An original specimen from the early finds of precious opal discovered in Ethiopia during the mid 1990s. From Yita Ridge, Mezezo, North Shewa Zone, Amhara Region, Ethiopia, it measures 29x21x9 mm and weighs 3.53 g.

Please note: this particular item is very small and is classed as a thumbnail-sized specimen (1-3 cm).

Yita Ridge is a locality where opal nodules are mined from rhyolitic rock. Opals from here, called chocolate opal because of the chocolate colour inside the nodules, came on the market in the early 1990s. Mezezo, the centre of an opal mining district, is a small town in North Shewa Zone. It lies at 2,476 m and is located 64 km NE of the city of Debre Berhan in central Ethiopia. In turn, Debre Berhan is located about 120 km NE of the capital Addis Ababa. North Shewa takes its name from the kingdom or former province of Shewa.

Shewa Opal is a type of opal-AG found in the Amhara Region of Ethiopia. Although it has been reported that Northern African opal was used to make tools as early as 4,000 BCE, the first published report of gem opal from Ethiopia appeared in 1994 with the discovery of precious opal in North Shewa Zone, Amhara Region. This opal, found mostly in the form of nodules, was of volcanic origin and found predominantly in stratified igneous rocks such as rhyolite, tuff, and ignimbrite. Opal from this area occurs in a wide range of body colours. Much of the opal has a brown, red, or orange body colour; however, yellow, white, and clear body colours are also found. A lot of the opal that was first discovered with a dark brown colour had crazing problems, but stable material did begin to enter the gem and jewellery market. These better quality opals from the region are often called Shewa opals or Mezezo opals after their locality of origin. However, the most significant opal discovery in Ethiopia to date was made in 2008 approximately 180 km (112 miles) north of Shewa, near the town of Wegel Tena in North Wollo Zone. This opal was different from the previous Ethiopian opal finds in that it more closely resembled the sedimentary opals of Australia and Brazil, with a light background and often vivid play-of-colour flashing from a body colour of clear, white, yellow, orange, or brown. Much of the opal from North Wollo Zone is produced from a single area of stratified volcanic rocks. The main vein is an opal-rich rhyolitic ignimbrite up to one metre thick that overlies a base of clay. The opal likely formed as silica-bearing waters accumulated on top of the impermeable clay. Silica gel precipitated in the pore spaces of the ignimbrite and was later transformed into opal. A third deposit was found in 2013, again in North Wollo Zone, but about 100 km (62 miles) north of the Wegel Tena area. Much of the opal in this deposit has a translucent grey to black body colour. It occurs in a seam of mineralised ignimbrite. North Wollo Zone opal, more commonly referred to as Welo (or sometimes Wello) opal, has now become the dominant Ethiopian opal in the gem trade.

Opal (a mineraloid) is a hydrated amorphous (i.e. without a clearly defined shape or form) type of silica (SiO₂·nH₂O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Although it is still regarded as a valid mineral species for historical reasons, opal is not a true mineral in the accepted sense of the word as it is either composed of cristobalite and/or tridymite or composed of amorphous silica.

Opal is classified into four types and transitions between Opal-CT, Opal-C and Opal-AG are common:

- Opal-CT: contains cristobalite-tridymite

- Opal-C: contains cristobalite

- Opal-AG: amorphous (Amorphous-Gel) (closely packed amorphous silica spheres from a diffraction grating to create precious opal).

- Opal-AN: amorphous (Amorphous-Network (found as hyalite).

There are two broad classes of opal: precious and common. Precious opal displays play-of-colour, common opal does not. Play-of-colour (or opalescence) is defined as ‘a pseudochromatic optical effect resulting in flashes of coloured light from certain minerals, as they are turned in white light’. The internal structure of precious opal causes it to diffract light, resulting in opalescence. Depending on the conditions in which it formed, opal may be transparent, translucent or opaque and the background colour may be white, black or nearly any colour of the visual spectrum. Mohs hardness 5.5-6.5.