Tortoise shell guitar picks have a reputation of mythic proportion.
Some players with stellar skills attribute their sound to these picks and
won't use anything else, despite the 1973 ban on the manufacture of
products made from the hawksbill turtle. Some of us have old tortoise
shell picks produced before the ban. I have one 
but I'm never comfortable using it
even though it's legal (note the faded D'Andrea logo in the middle), The
more tortoise picks are used, the more their mystique grows and the more
likely it is that turtles will be slaughtered so that tortoise shell
picks can be bought
and sold on the illegal
market. Because of these concerns, the pick I used most often 
was a "three for a dollar" 0.96 mm
large, rounded plastic triangle (346 shape).
Still, over the course of lifetime, 
I've probably spent more on picks 
than others have spent on an entry
level guitar 
I've been a
sucker for picks that are "just like tortoise", not so much because I
wanted tortoise, but more because I wanted to see if anyone could
duplicate it! Until recently, they couldn't. The husband of one of my
students, who holds a masters degree in materials science from MIT,
explained that the problem with duplicating tortoise was that the
structure of living tissue was highly irregular and could not easily be
manufactured.
John Greven has made a
mission of duplicating tortoise shell. His original Tor-tis picks, made by
TurtleWorks, look good and have a very nice sound--so nice that I have a
bunch of them 
--but, like all
of the faux tortoise shell picks, they don't sound like tortoise.
Last year, rumors started circulating that he had found a new formulation that really did sound like tortoise. Yeah, yeah, sure, sure. Like we hadn't heard that before. But hope springs eternal, so I plunked down my $15* (now$20) to give one a try. Lo and behold, he's done it! (My student's husband was right. According to Greven, "This new material is a polymerized protein that is very close chemically to the real thing". It's grown!)
These picks have made me rethink my opinion of tortoise shell picks. When the only source was the illegal market, I'd convinced myself that while tortoise was different, it was not necessarily better than other materials. Now that the new Tortis picks have removed the moral and ethical dilemma, I have to admit that these picks are far and away better than anything else I've used. They give me cleaner sound and an amount of volume control that I was never able to achieve with plastic.
Even at $20 each, I've found myself ordering a few spares "just in case". If for some reason the new Tortis picks were to become unavailable, I still couldn't use tortoise, even my old ones, for reasons I've already stated, but now I'd know what I'd be missing.
Red Bear Trading Company is the sole supplier of the new formulation Tortis picks. If you've always wanted to give tortoise a try, click on the link for a legal, guilt-free experience.
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*If $20 sounds like a lot of money for a guitar pick, well, that's because it is! However, players who use tortoise say $20 is cheap compared to current price of around $50, if they can be found at all.