Opal is basically Silica with up to 10% of water (yes - water!)
Opal is neither hard nor tough. It is only 5.5-6.5 on the hardness scale and may easily be broken as it is also brittle. However, the main problem with Opal is that it is prone to dehydration. This causes some stones to "crack", or as it is termed "craze".
Very competent scientists are working to stabilise craze-prone Opals and to repair those which are already crazed. However, crazing is currently still irreversible. Once a stone starts to craze, the condition of the stone will only get worse.
What you may be able to do is to get a jeweler to see if he can contact a specialist to polymer impregnate your Opal to stop further crazing and to repolish it so as to disguise the chip. This may cost as few people opt to "repair" their Opals. You may find it more economical to buy a new Opal for your ring, especially if it is of commercial quality.
Hope that helps! (P.S. I'm sorry about your Opal! The person who sold it to you should have let you know how to take care of it...What a shame...)
I LOVE gemstones and I teach gemology.