Topaz is a category of gemstones similar to the mineral species 'Quartz'. However, a Topaz is different from Quartz. Topaz (having hardness of 8 on Moh's Scale) is harder than Quartz (having hardness of 7 on Moh's scale). It is available in different colors like - Blue, Yellow, White, Pink, etc.
Astrological significance: Blue Topaz is considered as a substitute for Blue Sapphire & hence can be worn by people born in the lagnas of Saturn (Shani) which are Capricorn (Makar) & Aquarius (Kumbh). Yellow Topaz is considered as a substitute for Yellow Sapphire and hence is suggested for people born in the lagnas of Jupiter (Guru) which are Sagittarius (Dhanu) & Pisces (Meen). Apart from these, topaz generally has healing qualities also.
For more details about Topaz, you may read our blog section here.