Topaz is category of gemstones made out of the mineral species 'Quartz'. 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.