??????

    Actually that's a complicated set of questions.  There is normally a
shallow crevice between the tooth and gum (the periodontal sulcus).
When this becomes inflamed and/or deepened we call it a periodontal pocket.
    Generally you don't want any restorative material within about 3 mm or
so from the bone.  This means for most people you don't want to extend
any restoration more than maybe 1 mm at most under the gumline.  If the
restoration is further under the gumline than this, anyone will show
some inflammation as a result.
    Having said that, some materials will tend to provoke less inflammation
than others under the gumline.  There are probably papers you could
google about this; my (sometimes faulty) memory is that on average
highly-polished gold is the most biocompatible in this regard, followed
roughly by glazed porcelain, amalgam, and resin.  These may be
distinctions without a real difference though, since any can cause
inflammation.  It also goes without saying (though I will) that a
smoother surface (whatever the material) will be less of a plaque-trap
and will cause less inflammation.  Also, some individuals may be
sensitive to one or another component of a restorative material.  For
example, in the early days of non-precious casting alloys, nickel was
often used, which is commonly sensitizing.  For that reason, it is
rarely used anymore to my knowledge.

Steve