Shagreen patch

I have a shagreen patch due to TSC. This is a strangely shapped and bumpy lump of tissue on my lower back. It is a classic sign of TSC. I have lived with the shagreen patch my whole life. It doesn’t bother me. it is another thing that is just curious to me.

One of the doctors knew that I had TSC because of the shagreen patch on my back because they checked me over to see what was going on. The doctors discovered that I have TSC. I don’t understand why it is called the shagreen patch. I also don’t understand why the shagreen patch feels bumpy when I touch it or feels bumpy on my back.