Not worth registering at all imo. "Howlite" is not a popular mineral/stone.. and there are only a small handful of domains even regged with it in the singular (proper) way.. about 10 or so across the whole internet. Aside from it being little known, its singular/plural is one and the same "howlite" would be both, there is not an "s" version for the plural.
An easy way to view this is to relate it to other gems, metals, etc.. that you're probably already familiar with.. take "gold" for example:
If you wanted to buy a piece of gold (singular) you would say, "May I buy a piece of gold?" and when you want to buy multiple pieces of gold (plural) it is still called "gold". You might say, "May I buy 7 pieces of gold?" But you would not say "I'd like to buy 7 golds."
Hope that helps, good luck.