Words That Don't Rhyme
There are only four common words in the English language that are nearly impossible to rhyme. What is interesting about them is that three of the four are colors, silver, orange and purple. Perhaps this is because colors demand a certain amount of originality in their naming. Note how modern crayon color namers feel compelled to issue interesting names for their creations. Recent additions to the Crayola stable of colors include, "inch worm, jazzberry jam, mango tango and wild blue yonder".
The other common word that does not have a rhyme is month. This is an Old English or Germanic word that derives from "moon", a month being roughly one cycle of the moon's phases. It was rather inconvenient for the ancients that the moon's phases do not divide evenly into the Earth's orbit around the sun. This forced them to adopt a rather artificial and somewhat unwieldy method of tracking time. Perhaps this is why we have held on to this singular, "un-rhymable" word.
Some other, less common words that have no rhymes are angst and scalp. There are also a whole raft of words ending in "-th", but some of these are number-places and thus are rather odd constructs, eighth, ninth and twelfth. There are also breadth, depth and width.
An interesting exercise in a classroom is to assign students a project of writing a poem using "silver, purple, orange and month". They will do well not to use the words in rhyming couplets. It is also a fun poser for friends and family. If you are ever confronted with the necessity of coming up with a rhyme for any of these words in a poem, the best tactic is to make up a name for a character. Dr. Suess was a master of this technique and many of his fanciful figures can be directly traced to the need to make a stanza fit a rhyme scheme.