What Do You Call Textured Envelope Seals?
It sounds like you are describing an embossed sticker.
There are a lot of services that can custom emboss stickers for you so you can get your own personal 'seals' as you referred to them, to close envelopes or put on certificates and things. 

These are common for companies and individuals to use when they want to make something look fancy. Papyrus includes an embossed sticker with every card they sell to give them a more expensive feel.