If you're farming Amarast, consider using the Advanced Nosam Cutter over the Sunpoint Plasma Drill when mining in the Vallis. The Sunpoint Drill can mine Thyst and Zodian which will dilute the chances of getting Amarast.
Specific mining laser type producing specific resource results are inconclusive. RNG applies here as well, making a resource drop chance booster a more efficient way to acquire the rarer resource type above.
Caves on the Western edge of the map seem to produce an adequate amount for crafting most things, as noted by several users in the comments of this article. The caves were noted to have orokin ruins and terrain pieces in them, so the higher amount of Amarast in these locations is possible due to that. Southwestern and Northwestern caves have been identified as the best locations.
Amarast is likely a portmanteau word based on amaranth, a purple-red flower, and amethyst, a purple quartz gem.