Are you sure it didn’t just always look like that? How well do you know your stone?
Before giving a couple reasons as to why your crystal changed etc please remember that crystals have many things going on inside of them, we may not always notice right away.
One way to get familiar is to look at your crystal in the sunlight. Often what is inside of them is not visible until they are held up in natural sunlight.
Crystals are affected by the environment in which they are kept. The amount of light they are exposed to, the temperature of where they are stored and the humidity can all affect them to one degree or another.
For example if you place a coloured crystal in sunlight for prolonged period you will find that its colour fades over time. I learned this the hard way with a beautiful piece of Fluorite.
-The exchange of energy between you and the crystal:
When you work with a crystal, particularly on a regular basis there is an exchange of energy that happens between you and the crystal. This intertwining and melding of your energies can leave its mark on your crystal.
This may result in a visual change, a change to the way it energetically feels or a change to the way it physically feels.