There are two ways to earn Sunstone.
One is by earning the score specified for the medal grade in Purenexus (e.g., if Purenexus is silver, the Nexus medal for that stage is gold).
(For example, a score equivalent to a bronze medal in Purenexus earns one Sunstone.
A score equivalent to a Gold Medal in PureNexus earns 3 Sunstones).
Another additional Sunstone is earned “when you score even higher than the highest graded Nexusmedal score”. This can be duplicated and earned on both BestScore and PureNexus.
(Often referred to as OverScore on Discord and other communities).
For example, if you hover over Chapter1-1 Tactical tutorial on the stage selection screen, the highest grade, Nexusmedal (Diamond), has a score of 10,000,000 points, with a Sunstone icon and the notation “+1,500,000” above it The score of Nexusmedal (Diamond) is 10,000,000 points.
In this case, for example, if your best score is 13,000,000 points, you are “3,000,000 points over the highest Nexusmedal score of 10,000,000 points” and “earn an additional Sunstone for every 1,500,000 points you exceed.
(Even if you already have the highest Nexus medal, you can update your score further by starting over with a large number of Sunstones and Nexus medals. That will give you additional Sunstone by OverScore, and so on.)