What are these effects supposed to replicate ? Is it a reflection, a refraction, caustics ?
When you do photography you often reflect a Softbox (a square diffuser that makes the light more even) or an umbrella (also diffusing the light but gives a round reflection). When taking pictures of shiny objects these are often placed so they reflect in the object, together with white and black sheets of papers around setup to give a good 3D feel representing the true shape.
These glass buttons simulate that effect, the Softbox reflecting in the surface from top. As the even light isn’t perfectly even the light is typical represented as a gradient where the edge of the softbbox is not so bright as the center of it.
The inner light in those button simulate both refraction and caustics.
Here’s a walk-through how real-world shiny objects are photographed, the principle applies with glass (I add it here to show the principle):